A twin scandal has broken here, simultaneously discrediting a national newspaper and raising questions about alleged bribes from Leyland, Britain's lone domestic auto maker.

The Daily Mail, a well-established, Conservative newspaper, launched the affair on Thursday with a sensational scoop covering its entire front page. It centered on a letter that indicated top executives of Leyland had received clearance from the Labor government for illegal payoffs to promote sales abroad.

But 48 hours later, the Mail's scoop turned to ashes. A financial analyst at Leyland. Graham Barton, admitted to Scotland Yard's forgery squad that he had faked the startling letter. "My enthusiasm got the better of me," Barton said. "But I wanted to get it out in the open. I feel very strongly about slushing."

David English, thd Mail's editor, has now apologized to his 1.3 million readers, Leyland and the government. As other papers and several politicians have observed, however, English has left unanswered the central questions: How could a professional newspaper run so crude a forgery, and why did it make no more than token, eleventh-hour efforts to check with the letter's supposed author and recipient.?

Analyst Barton's admission, moreover, will now enable the government to turn attention from Leyland's alleged payoffs to the Mail's misconduct. This will bury what might have been another object lesson in how large corporations do business in the Third World.

Tonight, Leyland is strenuously denying that its own hasty investigation has turned up any evidence of bribery of corruption. It does not deny that Barton did prepare an internal report on "special accounts."

According to other, still unchallenged documents, these were outsized commissions paid to agents and middlemen, enabling them to avoid foreign taxes and exchange controls in return for opening up markets to Leyland in the Middle East, Africa and elsewhere.

Nor has there been any denial that Leyland budgeted just sort of $20 million for "special payments" in the accounting year of 1975-76

All of this material is now dwarfed by the faked letter which looked so incriminating when it first ran in the Mail. It was supposedly written by Lord Ryder, a former newspaper executive and now chairman of the Nation Enterprise Board, the state holding company that owns 95 per cent of Leyland. It was supposedly sent to Alex Parker, Leyland's Chief Executive.

The most damaging line, boxed in boldface type by the Mail on its front page Thursday, said:

"I must inform you that the proposed method for dealing with 'special account arrangements has now been nodded through by the secretary of state."

At one blow, as the Mail triumphantly noted in its story, the Conservative organ had struck a deadly blow at nationalized industry and the ethics of the Labor government. The "nodding" secretary of state could only have been Eric Varley, the industry minister, who is in charge of Ryder's Enterprise Board.

As one indignant member of Parliament, Ian Wrigglesworth, said today:

"A child with a Sherlock Holmes kit could have found out that this was a forgery."

The letter from ex-journalist Ryder contained these misspellings. "Received," "fundamentaly" and "dangerous". They were so barbarious that the Mail's first-day account of the letter corrected two of them. In addition, Ryder's purported letterhead referred to him as a "baronet" when he is in fact a "knight", the sort of mistake that is not usually made by members of the British Establishment.

Editor English said he had indeed, noted the discrepancies but received satisfactory explanation from someone.He said he, tried, the night the story was in print, to reach Lord Ryder and Alex Parker, the supposed recipient, but had failed.

Two Sunday papers here have said that English paid cash for his documents. He characterized as "ridiculous" one estimate of $85,000.

Just three years ago, the paper splashed on its front page. "Who Forged Wilson's Signature?" under an announcement that "the Daily Mail follows its revelations of land speculation by Labor Party members with anextraordinary development . . ."

The answer to its question was that the paper's own informant, one Ronald Milhench, had forged Wilson's signature. Milhench got three years and the Mail was sued for libel by Wilson.

Both Prime Minister Callaghan and Minister Varley are saying that the government will not tolerate bribery abroad. Callaghan has just presided over a summit meeting that denounced the practice.

In fact, there has been a permissive governmental attitude toward slush funds for foreign sale as that is bipartisan. Just last year, Joel Barnett, the Treasury's cheif secretary explained to the House of Commons that there would be no inquiry into the $6 million that Shell and British Petroluem admitted giving Italian politicians.

It is up to the host country, Barnett said, to enforce ethical conduct. "The high standards of business practice that we expect from British companies do not apply everywhere", he said.

Barnett was unmoved by the fact that 68.9 per cent of BP is owned by "the British government itself