The Internal Revenue Service announced on Wednesday it is postponing the country’s tax-filing deadline until May 17, as the agency grapples with a mounting backlog of 24 million returns awaiting processing since the 2019 tax year.

The workload has put the agency underwater — and under political siege — as lawmakers fret that long-unresolved troubles at the IRS could undercut the Biden administration’s economic recovery efforts. Millions of Americans have not received some stimulus checks under prior coronavirus aid packages, even as the tax agency continued distributing payments Wednesday under the $1.9 trillion stimulus signed into law this month.

The IRS shared the full scope of its backlog with the House Ways and Means Committee and the agency’s own watchdogs in recent days. The numbers, obtained by The Washington Post, dwarf the data the IRS has shared with the public. Explaining the postponement, IRS Commissioner Chuck Rettig said in a statement the government is doing “everything possible to help taxpayers navigate the unusual circumstances related to the pandemic, while also working on important tax administration responsibilities.”

The effects of the IRS backlog already have been substantial: The delays have kept some Americans from receiving their tax refunds for months while preventing some cash-strapped workers and companies nationwide from taking advantage of some of the stimulus benefits that Congress authorized to blunt the economic impact of the pandemic.

For Patrick O’ Conor, the IRS backlog has been costly: He estimates that the government owes him about $16,000 in tax refunds and stimulus payments as a result of significant lags in processing his 2019 and 2020 taxes. The Frederick, Md.-area resident said it has been particularly rough because he and his wife recently had a baby and bought a home in the past year, and they need the money to cover their new expenses.

“We haven’t missed any payments yet on anything,” he said, “but it’s come really, really close.”

The delays threaten the IRS’s ability to deliver an array of new relief under the $1.9 trillion American Rescue Plan, and could result in uncomfortable questions for Rettig, who is set to testify before the House Ways and Means Committee on Thursday. In recent weeks, some congressional Democrats and Republicans had urged the IRS to push the tax-filing deadline from April to later this year, similar to the filing deadline for 2019 returns. Some lawmakers celebrated the decision Wednesday.

“This extension is absolutely necessary to give Americans some needed flexibility in a time of unprecedented crisis,” said Rep. Richard E. Neal (D-Mass.), the panel’s chairman, and Rep. Bill Pascrell Jr. (D-N.J). “Under titanic stress and strain, American taxpayers and tax preparers must have more time to file tax returns. And the IRS itself started the filing season late, continues to be behind schedule, and now must implement changes from the American Rescue Plan.”

For some lawmakers, the IRS’s troubles come as no surprise. Under GOP leadership, Congress slashed the agency’s budget by billions of dollars, contributing to the loss of tens of thousands of critical IRS jobs, while leaving long-known deficiencies in its computer systems unaddressed for decades.

The pandemic has brought additional challenges, forcing much of the IRS workforce to complete its tasks from home and saddling the agency with new responsibilities, including three rounds of stimulus payments. Most of these payments have reached hundreds of millions of Americans without too much difficulty, making them one of the government’s most popular relief programs.

But the agency has struggled in other respects, including in its work over the past few years to process tax returns from individuals and businesses alike. The details are laid bare in information the IRS provided to the House Ways and Means Committee as well as the Treasury Inspector General for Tax Administration. A spokesman for the inspector general this week confirmed the figures but said they had not independently audited information it obtained from the IRS for accuracy.

The total backlog appears to stand at more than 24 million filings, the data show. That includes more than 12.3 million paper-based tax returns the IRS has received, but hasn’t started processing over the past two tax years.

Most of these filings are from businesses, but the tranche also includes 2.4 million returns from individuals reflecting their 2019 earnings. As a result, these Americans may not have received stimulus payments under the relief bill Congress adopted in December, since the stimulus law tied their eligibility for checks to their 2019 taxes.

The IRS cautioned that not every individual in this group may have been eligible to receive the aid, adding that it did not have data on the numbers of Americans still waiting for the earlier stimulus payments. But Ken Corbin, the commissioner of the wage and investment division at the IRS, defended the agency’s work getting stimulus payments out and said the number of checks sent matches “what was projected to be the eligible population.”

“The IRS will always have returns in processing,” he said, citing the impact of the coronavirus and other more recent obstacles, including inclement weather that slowed agency operations.

These 2.4 million Americans still can collect previous stimulus payments as part of their 2020 returns, if they are eligible. But they may face another long wait. The IRS has racked up an additional backlog of 12.4 million returns filed mostly by individuals, both electronically and in paper filings, from the 2020 tax year that it has started processing but suspended pending further, deeper review, according to the government figures.

The tally includes about 7 million returns that the IRS has designated for “error resolution,” meaning they will require manual review that takes months to wrap up. The Post first reported on these figures last week. But the IRS has millions of additional tax returns to analyze, including those involved in investigations related to issues such as identity theft, contributing to its immense workload.

The backlog has only compounded other troubles at the IRS, including technical glitches that took down its online portal for tracking refunds earlier this year — and lingering staff shortages that appear to affect its ability to answer taxpayers’ questions in a difficult year. Only about a quarter of those who call the IRS end up speaking to someone, according to agency data shared with lawmakers, who expressed concerns about the response rate in what is shaping up to be a complicated tax year.

The ranks of the frustrated include Neava Ford, a Kansas City, Kan.-area resident who filed her 2019 tax returns late in the year, yet hasn’t heard much from the agency five months later. She hasn’t received a stimulus check, either, but Ford says she hasn’t bothered to call the IRS because she knows she’s unlikely to reach anyone.

“I knew it would be pointless with everything that would be going on,” she said.

Acknowledging the issues, the Treasury Department said in a statement Wednesday that the backlog reflects “serious challenges stemming from inherited problems and diminished capacity.” An agency spokeswoman said the IRS so far has sent out 90 million stimulus payments under the American Rescue Plan. “It will take time to work through these challenges we inherited, but this investment will help us in tackling them head on,” the department added.

The agency’s struggles have generated bipartisan concern on Capitol Hill over the past few months. In February, for example, Republicans on the House Ways and Means Committee — including Rep. Mike Kelly, a member of its oversight panel — wrote the IRS on behalf of constituents who have contacted lawmakers’ offices, trying to track down their refunds.

“We cannot have millions of hard-working Americans and small business owners waiting up to a year to receive money that they are owed, and the federal government paying billions of dollars in interest on top of that,” said Kelly, adding that his office never received a response to the earlier letter. “This needs to be fixed.”

Rep. Kevin Brady of Texas, the top Republican on the House Ways and Means Committee, added that the new tax deadline also serves as a sign that the $1.9 trillion stimulus has caused “chaos, to the point that delaying tax filings in order to accommodate the law’s provisions may be a wise decision.”

Democrats, meanwhile, have said the situation points to the need for additional changes at the IRS. The recently adopted American Rescue Plan includes billions of dollars to help modernize the agency, but party lawmakers say a more permanent fix is necessary to prevent similar backlogs in the future.

“It’s much harder for the IRS to build the plane while flying it,” Sen. Ron Wyden (D-Ore.), the chairman of the Senate Finance Committee, said in a statement. “We need sustained funding so the IRS can build and maintain these systems over the long-term.”

correction

The 2020 tax filing deadline has been pushed back a month to May 17. A previous version of this story reported an incorrect date for the new filing deadline.