Despite President Reagan's pledge that the pending INF treaty will be based on ''the most stringent verification regime'' in history, an authoritative CIA study rules out any way to prevent Soviet cheating, at least during the first few years of the treaty.

The intelligence study was completed a year ago and has been held under tight secrecy since. Senior officials at the State and Defense Departments and on the National Security Council staff who support the president's drive for the INF treaty have read the study -- and wept. It flatly rejects the possibility of verifying compliance with any treaty that commits the Soviets to limit or destroy mobile missiles such as the SS-20, as the INF treaty nearing final agreement in Geneva will do.

Rather than heed this warning and modify his expansive guarantee of strict verification, Reagan has been persuaded by architects of the new de'tente to embellish his claim. But instead of repetition chasing away the verification nightmare, it promises to confront him on the Senate floor after the treaty is signed at next month's summit.

On Dec. 11, 1986, a high-level interagency ''working group'' reported to then CIA director William J. Casey in the careful words of the intelligence bureaucracy: ''our current capability to meet adequately the demands placed upon our resources to address effectively the mobile missile problem is limited.''

Those words highlight the final report of Casey's supersensitive Intelligence Requirements and Analysis Working Group. Set up to scrutinize verification in monitoring mobile missiles in the Soviet Union, the group struck at the heart of next year's INF ratification fight in the Senate. ''Future requirements'' for verifying compliance with a mobile-missile agreement such as the INF treaty, it warns, ''are likely to be stringent and may involve accounting for nondeployed'' -- that is, hidden -- ''missiles.''

There is no conceivable way today to account for nondeployed SS-20s. After the estimated 441 missiles in launchers are destroyed, senators and administration officials who endorse the INF treaty estimate the Soviets will have between 145 and 530 ''nondeployed'' SS-20s left. But defense hard-liners estimate there could be as many as 2,000. However many there are, the missiles are stashed out of sight, absolutely beyond discovery -- unless the Kremlin chooses to uncover them.

A high administration official, worried about the effect on fence-sitting senators of inability to verify, warns privately that Reagan's blanket statements will be self-defeating when all verification shortfalls become known. ''The president is saying ironclad and airtight verification, but that's dangerous to say because it's impossible,'' he told us.

The ''data base'' of SS-20 missiles is supposed to answer these questions: How many SS-20s do you have? Where are they located? Can U.S. inspectors see them? With Mikhail Gorbachev due here in less than three weeks, the administration does not have the answers.

Instead, compliance cannot be ensured for from five to seven years, when the hidden SS-20s not destroyed will begin to ''wither away'' as unreliable -- and hence useless -- for lack of testing. But even that surmise depends on Soviet compliance with a proviso in the INF treaty forbidding future SS-20 testing. Intelligence officials believe the Soviets might beat this by testing their unacknowledged SS-20 stockpile on launchers of long-range SS-25s, a new missile with similar characteristics but which is permitted under the treaty.

According to the CIA study, the United States is ''hampered'' by the lack of a ''uniform data base of information'' on mobile missiles, raising an ominous question about Soviet exploitation of hidden mobile missiles for possible surprise attack. ''A true capability to locate, identify and track mobile missiles for the purpose of targetting is evolutionary,'' says the study, requiring ''significant enhancement over present capabilities.'' That rules out any way today to knock out mobile missiles because their movement on the ground cannot be tracked.

The president's verification promise puts a weapon into the hands of anti-INF senators. No longer do they have to prove the dubious thesis that Gorbachev is secretly plotting a surprise nuclear strike with hidden SS-20s. Whether or not the vote is even close, those senators can embarrass the president by challenging him to prove the CIA wrong when it reported ''stringent'' verification procedures are impossible.