BRITAIN and the United States were locked in a deepening diplomatic dispute last night over Washington?s failure to implement a mutual extradition agreement that Parliament approved three years ago.

Congress?s refusal to ratify the deal means that Britain cannot prosecute criminals residing in the US, including sex offenders and paedophiles.

America has sought the extradition of 44 alleged offenders since 2003, and 12 have already been sent to the US. The extradition of several others has been approved but is awaiting appeal. In contrast, Britain extradited only five suspects from the US in the same period under the previous thirty-year-old extradition treaty, including just one in 2005.

Ministers have repeatedly raised the issue with their US counterparts, and Charles Clarke, the Home Secretary, protested again when he met Alberto Gonzales, the US Attorney-General, in London yesterday.

Diplomats led by Sir David Manning, Ambassador to the US, have also held about 30 meetings on the issue on Capitol Hill in recent months. But pleas have fallen on deaf ears. Indeed, America?s failure to abide by its side of the deal is fast becoming a political embarrassment for a British government often accused of showing undue deference to Washington.

A total of 145 MPs, including many Labour backbenchers, have signed a Commons motion demanding that extraditions to the US be halted until the Senate ratifies the treaty.

A Home Office spokeswoman confirmed that it was encountering obstacles in extraditing suspects, particularly sex offenders, from the US. This is because the existing treaty, dating to 1972 and listing a schedule of extraditable offences, ?was drawn up before the internet was invented?. She said: ?There are people we want who are successfully evading justice.?

A senior US official told The Times yesterday that the Administration was sympathetic to Britain?s concerns and hinted that a fresh attempt to force the treaty through the Senate would be made within weeks.

The official said that the problem had been caused by the influential Irish-American lobby, which is worried that the treaty could be used to pursue IRA suspects. Senators are worried about alienating any section of voters before November?s congressional elections. British diplomats have emphasised that, after the Good Friday agreement, Britain has little interest in pursuing IRA suspects.

Those currently facing extradition to the US under the 2003 deal include the ?Nat West Three? who face up to 35 years in prison if convicted of Enron-related fraud charges. Their lawyers, who are appealing to the House of Lords, claim they are victims of an unequal law.

Another Briton extradited to the US for financial offences is Nigel Potter, former chief executive of Wembley plc, who will today begin an appeal against a three-year sentence for fraud.

Potter was convicted of conspiring to bribe a US politician so that his company could install fruit machines at an American greyhound track. No bribes were paid, however, and Potter was acquitted of all but three charges of wire fraud.

In contrast to the white collar suspects being sent to the US, Britain is being prevented from bringing to justice US suppliers of computer child pornography, including some of those related to Operation Ore two years ago, which discovered 6,000 Britons using an American website.

The 1972 treaty also contains a statute of limitations which does not allow extradition for offences that took place more than six years ago. Officials say this is a particular problem in child abuse cases because victims often do not come foward until they are adults.

Since the new treaty was agreed in March 2003, countries such as Lithuania, the Marshall Islands, Micronesia and Peru have had extradition deals ratified by Congress. John Ashcroft, the US AttorneyGeneral in 2003, said that the new treaty ?should serve as a model to the world for successful and efficient co-operation.?