I know this reads like the plot of a Tom Clancy novel; but Washington sources insist it’s true.

George Mitchell’s rumored appointment as President Clinton’s secretary of state, they say, has been put in deep jeopardy because of a covert plot by a British intelligence agency to undermine peace negotiations in Northern Ireland.

The Mail on Sunday, a British tabloid, reported Sunday that MI5, the British agency comparable to America’s Secret Service and Federal Bureau of Investigation, observed Mitchell’s top aide, Martha Pope, sneaking away for “weekend trysts” with Gerald Kelly, a former Irish Republican Army bomber. A file detailing those meetings reportedly was sent by MI5 to the White House and British Prime Minister John Major “about three weeks ago.”

The New York Post, which ran photographs of Kelly and Pope under a large headline declaring “Sex Scandal Perils Peace Talks,” speculated that the controversy might “hurt Mitchell’s chances” for the secretary of state’s job. A Post reporter said he was upset at the way his editors played the story. The journalist said there is a strong possibility that MI5 had engaged in a bogus “smear” against Pope to weaken Mitchell’s hand as he returns to the Northern Ireland peace talks next week.

A Republican member of Congress, Rep. Peter King of New York, jumped to Mitchell’s defense.

“This is typical MI5 propaganda. It’s an act of desperation by the British to hide the fact they’re responsible for a breakdown in peace talks,” King said.

A foreign policy aide to former Secretary of State Edmund S. Muskie said there could be a “backlash” within the president’s inner circle against the British government for seeking to “influence the appointment” of America’s top diplomat. Relations between the Clinton White House and John Major’s conservative British government have always been rocky. Former Reagan-Bush political consultants helped Major survive his 1995 re-election. George Stephanopolous, a top Clinton aide, has said he will hire out to Major’s Labor Party opponent in the coming British elections.

On the other hand, the former Muskie aide said the media flap over Pope could make Mitchell a bit too controversial for a White House already mired in tabloid scandals. That could tilt the president toward United Nations Ambassador Madeleine Albright, or National Security Council adviser Anthony Lake. The choice of Albright would please feminists, who are pressuring the president to reward women for their overwhelming support in last November’s election. Like Mitchell, Albright is a Muskie protege.

CNN reported Monday that Mitchell no longer is among the “top three” candidates for secretary of state. They were said to be Albright, retiring Democratic Sen. Sam Nunn of Georgia and Thomas Pickering, the former U.S. ambassador to Moscow. Pickering is a Bowdoin College graduate.

Sen. William S. Cohen, meanwhile, remains the clear front-runner to become secretary of defense for the Clinton administration’s second term. A new poll by the Fox Network indicated voters — by a 61-to-26 percent margin — liked the idea of a Republican in the new White House Cabinet. White House aides said an announcement on the state and defense Cabinet jobs could come as early as Tuesday or Wednesday.

Northern Ireland has been the graveyard for many political careers. Democrats here are hoping the troubled land does not claim George J. Mitchell, whose paternal ancestors migrated to the United States from the city of Cork in Ireland.

Two years ago President Clinton named Mitchell to head a U.S. drive to spur economic development in Northern Ireland, a region with an unemployment rate of more than 50 percent in some counties because of the bloody civil war that claimed more than 3,000 victims over the past quarter century. The success of that economic effort led to Mitchell’s appointment as independent negotiator in peace talks between Great Britain and the Protestant government of Northern Ireland. The result has been an off-and-on cease-fire between British troops and the IRA, which wants Northern Ireland to be rejoined to Catholic-dominated Ireland.

The Mail on Sunday reported that MI5 had Gerald Kelly under surveillance when it observed the former IRA bomber sneak away for “weekend trysts” with Pope, Mitchell’s former top Senate aide. After he became majority leader, Mitchell named Pope to be the U.S. Senate’s first woman sergeant-at-arms, which put her in charge of the political chamber’s large bureaucracy of support personnel. Pope, 51, was the woman who led Clinton and former President George Bush to the speaker’s rostrum before all speeches before joint sessions of Congress.

Kelly, 42, was described in media reports as “a dashing rogue” who was convicted of taking part in the IRA’s first bombing campaign against London in 1973. He escaped from Northern Ireland’s Maze prison in 1983, only to be recaptured 10 years ago. After his release from prison in 1990 Kelly joined secret talks with the British government as a member of Sinn Fein, the IRA’s political arm, that led to the 1994 cease-fire declaration.

The Mail on Sunday claimed that Kelly was “smitten” by Pope and addressed “romantic poetry” to her. Confronted by reporters in London, Pope denied ever meeting the IRA official. Kelly described the Mail’s account as “total rubbish.” Mitchell issued a statement saying the tabloid report was “totally false” and supported Pope’s assertion that she “never met Gerry Kelly.” The British embassy refused comment on the charges, as did White House press aides.

Ian Paisley, the fiery leader of Northern Ireland’s Protestant Unionist Party, called for Pope’s dismissal. He charged, “People in Senator Mitchell’s office are not to be trusted, for they’re friends of leading members of the IRA.”

Sources here said the allegations may have been more directed at Pope, who reportedly was viewed by British negotiators as being pro-Irish and the author of Mitchell’s report outlining a possible peace settlement.

“MI5 mustn’t have read the Mitchell report very carefully to be alleging that Pope was influenced by an IRA sympathizer,” said Jack Holland, the author of a book about MI5 operations in Northern Ireland. Holland told The New York Post that the six principles of the Mitchell report would cause the IRA to “cease to exist” if they were implemented.

Like former FBI Director J. Edgar Hoover, MI5 has a long history of leaking embarrassing disclosures to influence national politics. In 1963 the British spy agency broke the news that British Defense Minister John Profumo had engaged in orgies with prostitutes hired by a KGB officer, nearly toppling Tory Prime Minister Harold Macmillan’s government. More recently MI5 was believed to be the source of tape-recorded telephone conversations by Prince Charles and Princess Diana to their various alleged lovers. — WASHINGTON