Homeland Security officials have threatened to make it compulsory for all British citizens to apply for visas to visit the United States over fears about potential terrorist threats.

Any British citizen can travel to the US without a visa under a bilateral "waiver" scheme, as long as they plan to stay for fewer than 90 days.

However, Homeland Security officials had suggested forcing British Pakistanis to apply for US entry visas because they are perceived as a potential terrorist risk.

Sources say that when UK government ministers rejected singling out the Pakistani community as "politically disastrous", US officials threatened to make it compulsory for all British citizens to apply for visas.

Instead, Britain's security services offered to share all relevant background information on UK citizens of ethnic Pakistani origin flying to America in an effort to defuse a potentially major political and diplomatic embarrassment.

An insider told The Herald: "It was pointed out to the Americans that the red tape and delays involved in hundreds of thousands of visa applications would not only inundate their embassy and consular staff, but would also hit their tourist industry hard."

The move to put Britain's Pakistani Muslims under scrutiny via the visa process came from Michael Chertoff, the US Homeland Security Secretary. He is understood to be alarmed by the number of "clean skins" - the intelligence community term for suspects with no previous criminal record or known connection to terrorist organisations - involved in attacks in the UK.

The American fear is that British al Qaeda "sleepers" could travel separately to the US and then mount an attack from within.

The UK's ethnic Pakistani population makes 400,000 visits a year to their ancestral homeland and Pakistan's ISI security agency does not have the resources to monitor all of them.

A number, including July 7 bombers Mohammed Sidique Khan and Shezhad Tanweer, are known to have used the excuse of visiting relatives to slip away for insurgent training in camps run by militant Islamists in Waziristan along the Afghan border.

Home-grown jihadis have also begun to use Kenya, Tanzania and Madagascar - none of which requires visas from UK passport-holders - as staging posts for unrecorded trips to Pakistan.

As part of the proposed compromise deal with the US, ISI has agreed to monitor the movements of "people of interest" to MI5 or MI6, provided they are identified in advance.

A Foreign Office spokesman said: "We are in close touch with the US about entry clearance and they are aware of our view that changes to the visa waiver programme could cause economic damage to both countries without materially enhancing the security controls over immigration."

The US authorities had no comment to make yesterday but are understood to be reconsidering their strategy in view of British objections.