The Carter administration is slipping behind schedule in its efforts to hammer out a comprehensive tax-revision proposal by the middle of September as the President has promised.

Although the deadline still officially is September 15, planners say the unveiling date now has been postponed until the first week in October, and could be put off even longer, depending on last-minute complications.

Those close to the drafting say there are unresolved decisions in virtually every section of the proposed package, from treatment of business tax cuts to changes affecting deductions for individuals.

One major sticking point aas been the controversy involving budget director Bert Lance. Insiders say the President has been so preoccupied with the Lance affair he has not had time to make decisions on tax proposals.

Primarily, however, top Treasury officials themselves still have not decided what recommendations they plan to make on various components. The tax package is being described as "a mass of option papers."

The administration has promised key congressional leaders it will send the package to Capitol Hill at least two weeks before adjournament. That currently is set for October 21, but is expected to slide some.

A few strategists are urging the White House to postpone the package until after Congress goes home, both to avert any conflict with the pending energy bill and coprevent having the package "nitpicked to death."

Planners say the Treasury still is aiming for the pre-adjournment deadline promised to Congress. Top officials reportedly already have begun sending pieces of the package to President.

By far the most controversial area still to be resolved is what the administration should do about easing business taxes - a move virtually all sides agree is needed to encourage investment.

In last year's presidential campaign, Carter embraced the concept of eliminating the so-called "double taxation" of corporate income both as profits and as stockholder dividends - at a cost of $15 billion.

However, the idea has proved so problem-laden that more recently W. Michael Blumenthal, the Secretary of thee Treasury has hinted the package most likely will involve a "combination" of smaller business tax cuts.

Planners are considering a compromise that would combine a partial "integration" of the double tax with a small reduction in the corporate tax rate and a modest boost in the present investment tax credit.

The partial integration would be accomplished by allowing stockholders to claim as a credit against the taxes they owe a portion of the tax the company paid on the income it dispensed as dividends.

Even that proposal, however, is considered likely lto run into some opposition. Critics of the plan already have branded it a "welfare programl for the rich" because so much of the break would go to stockholders.

Other prposals that official say likely to go into the Carter tax package include:

An end to the long-standing preferential tratement for capital gains - profits from the sale of property or other investments. Only 50 per cent of the income from such sales now is subject to taxation.

The break would be eliminated for all such gains except profits from the sale of a taxpayer's personal residence. The provision has been a prime target of "tax reform" groups.

An across-the-board reduction in tax rates for all taxpayers, with the maximum rate for all taxpayer may be assessed trimmed to 50 per cent from the present 70 per cent for investment income.

A special Tax cut aimed primarily at low - and middle income taxpayers, accomplished by replacing the present $750-a-dependent personal exemption with a $200 Tax credit, benefitting the poor proportionately more.

A sharp reduction in the number of itemized deductions a taxpayer may claim - part of a broad move to simplify preparation of tax returns. The writeoffs would be replaced by an expanded standard deduction.

A crackdown on expense account deductions for business, including writeoffs now allowed for meals, travel and entertainment, and possibly those for club dues, theater tickets and convention travel as well.

Narrowing of the so-called "marriage penalty" under which couples pay more filing joint returns than they would if they remained single and lived together. One spouse may get an offsetting credit.

Repeal of the controversial tax break for domestic international sales corporations, which providws $1.2 billion a year in tax subsidies to exporters - a bonus critics say is unjustified.

The administration also plans to propose a crackdown on present provisions permitting multinational corporations to defer payment of taxes on income they earn from foreign sources - another target of "reformers."

Tax strategists say the administration is trying to assemble its package so that, when all these provisions are in place of, the overall proposal will produce no more than a $10 to $15 billion revenue loss.

Indeed, President Carter had been expected to trim the proposal even further to avoid upsetting his drive to balance the budget by fiscal 1981. However, insiders say it may be expanded if the economy flags.

For all its provisions, the Carter proposal still is likely to omit some favorite targets of "tax reform" groups, including a crackdown on real estate tax shelters and the new faster depreciation for business.

Nevertheless, spokesmen for these organizations say the outlines of the program that have susrfaced appear to point to a significant tax-revision proposal - certainly the most sweeping since 1969.

The question is, how the package will be received, both by Congress and business. The lawmakers generally have seemed cool to the idea, and some proponents fear the effort may suffer from the Lance affair.

The White House also may run into opposition on some issues from Arthur F. Burns, chairman of the Federal Reserve Board. Burns already has warned against eliminating the preference for capital gains.

Robert M. Brandon, head of Ralph Nader's Public Citizen Tax Reform Reserch Group, says the real test may be whether, after all the changes are made, a taxpayer still will be able to escape payment of taxes.

So for, most tax experts agree it is unlikely that the administration's proposal, when it comes, will be able to meet that standard. But Treasury planners say they are going to try to go a good bit of that way.