The move sets the stage for an explosive corruption scandal involving President Bush’s party.

Jack Abramoff, a lobbyist with extensive financial ties to numerous politicians — including Tom DeLay, the former Republican House leader — pleaded guilty to conspiracy, fraud and tax evasion, under a deal in which he is believed to have named high-profile politicians he bribed in a cash-for-favours racket.

Appearing in a Washington court, Abramoff agreed with Ellen Huvelle, the US District Court Judge, when she said that he had engaged in a conspiracy involving "corruption of public officials", and had provided campaign contributions, trips and other items "in exchange for official acts".

The plea agreement drawn up by the US Justice Department is believed to list extensive bribes, including cash, gifts, expensive meals and foreign trips, that Abramoff and his associates are alleged to have given politicians in exchange for legislative action favourable to the lobbyist’s clients.

"Words will not ever be able to express my sorrow and my profound regret for all my actions and mistakes," Abramoff told the court. "I hope I can merit forgiveness from the Almighty and those I’ve wronged or caused to suffer."

According to the charges, Abramoff and Michael Scanlon, a business associate who pleaded guilty last month to conspiring to bribe politicians, overcharged several Native American tribes by millions of dollars and used that money to shower golf trips, sports tickets, lavish dinners and other favours on politicians. The tribes had hired Abramoff to lobby politicians to get legislation favourable to their gambling interests.

Abramoff faced up to 30 years in prison, but under the deal the sentence will be reduced to between 9 and 11 years. He has also agreed to pay at least $25 million (£14.3 million) in restitution.

The investigation, which has been expanding for nearly two years, has been viewed as a time bomb for the Republicans.

Yesterday’s plea deal now makes the prospect of politically explosive criminal charges on Capitol Hill very real.

According to one unconfirmed report, Abramoff is believed to have thousands of e-mails that detail what politicians did in exchange for the money he sent to their campaign accounts.

Although Republicans have been hit by a series of sleaze scandals, including the indictments of Lewis "Scooter" Libby, former chief of staff of Dick Cheney, the Vice-President, and Tom DeLay, it is the Abramoff scandal that has made White House strategists the most nervous. Court papers in Scanlon’s plea last month say that he and Abramoff "provided a stream of things of value to public officials in exchange for official acts".

Scanlon’s papers, and Abramoff’s yesterday, referred repeatedly to an unnamed congressman, acknowledged by his lawyer to be Bob Ney, a Republican from Ohio. Mr Ney has been subpoenaed in the inquiry but denies any wrongdoing.

It is alleged that in addition to $14,000 in campaign contributions, Mr Ney also got a golfing trip to St Andrews. Among others who went on a paid-for trip to St Andrews was DeLay.

More than 30 members of Congress were found to have taken legislative action favourable to Abramoff’s Native American clients after taking money from the lobbyist or the tribes. Most are Republican, but include Harry Reid, the Democrats’ Senate leader, and Byron Dorgan, a Democrat senator from North Dakota.

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