Carter administration officials are revising their estimate of the fiscal 1981 budget deficit to somewhere around $45 billion rather than the $29.8 billion they projected last July.

The sharply higher estimate stems from expectations that Congress will cut taxes next year and that it will not approve two earlier White House proposals that together would have raised $6.6 billion in added revenues. The administration also is expecting total spending to run several billion dollars over budget. At the same time, the fact that unemployment is apt to be lower than originally forecast will reduce jobless payments by billions of dollars.

The higher projections for fiscal 1981 follow informal estimates showing the budget deficit for fiscal 1980 now is likely to wind up at between $61 billion and $63 billion -- making it the second-largest red-ink figure on record.

The highest budget deficit was $66.4 billion recorded in fiscal 1976. Fiscal 1980 ended at midnight Tuesday. The 1981 fiscal year began officially yesterday.

The higher fiscal 1981 projections were expected to add to the nervousness in the nation's financial markets, already buffeted by a tightening of money and credit policy and rising interest rates.

Although the fiscal 1980 figure is in line with expectations, the revision for fiscal 1981 is sharply higher than that forecast by the administration in July when the president sent his last formal budget estimates to Congress.

At that time, the White House was expecting a $29.8 billion deficit for fiscal 1981, following a red-ink figure of $60.9 billion for the budget year that ended Tuesday.

In the budget Carter submitted last January, the administration projected budget deficits of $39.8 billion for fiscal 1980 and $15.8 billion for the current fiscal year.

On Aug. 28, Carter proposed a new economic renewal plan that called for a series of major tax cuts that would increase the fiscal 1981 budget deficit by about $5.7 billion. That estimate since about $5.7 billion. That estimate since has been raised to $6 billion.

The other changes contributing to the higher deficit for fiscal 1981 include:

Congressional defeat of two key Carter revenue-raising proposals -- $3.4 billion from a gasoline and diesel fuel tax, and $3.2 billion from withholding taxes on income from dividends and interest.

Several billion dollars in increased spending from congressional decisions to overstep the president's budget on some key programs, plus $2 billion in higher outlays from Carter's own economic revitalization plan.