British police yesterday foiled a plot to blow up aircraft mid-flight between Britain and the United States in what Washington said might have been an attempted al Qaeda operation.

“We are confident we have disrupted a plan by terrorists to cause untold death and destruction,” said London police Deputy Commissioner Paul Stephenson. “Put simply, this was intended to be mass murder on an unimaginable scale.”

US Homeland Security Secretary Michael Chertoff told a news conference: “This operation is in some respects suggestive of an al Qaeda plot, but because the investigation is still under way, we cannot yet form a definitive conclusion.”

He said: “While this operation was centred in Great Britain, it was sophisticated, it had a lot of members and it was international in scope.”

While British police sources did not rule out an al Qaeda link, they played down the idea of direct involvement by the global militant group.

Britain and the United States both stepped up security, causing severe delays at airports following the announcement of the plot, which a police source said was believed to involve a “liquid chemical” device.

Unconfirmed media reports said anywhere from six to 10 airliners had been targeted in the plot.

The US Department of Homeland Security raised the threat level for passenger aircraft to “red”, its highest level, for the first time. US authorities banned liquids, including drinks, hair gels and lotions, from US commercial flights.

Britain’s security services upped the threat level in the country to “critical” from “severe”, the highest of its five ratings, which means “an attack is expected imminently”.

Interior Minister John Reid said police were confident the main players had been detained in raids overnight. Twenty-one people have been arrested.

The head of London’s counter-terrorist police, Peter Clarke, said the plot had “global dimensions” and Stephenson said British authorities were cooperating with foreign agencies.

<B>Al Qaeda threats </B>

The security alert comes 13 months after four British Islamist suicide bombers killed 52 people and injured about 700 on London’s transport network.

Authorities declined to give the nationalities of those arrested. A police source told Reuters some were British. Arrests were made in the capital London, the southeast of England and Britain’s second biggest city Birmingham.

Last month, al Qaeda called on Muslims to fight those who backed Israel’s attacks on Lebanon and warned of more attacks unless US and British forces pulled out of Iraq and Afghanistan.

Al Qaeda hijacked passenger aircraft in September 2001 to destroy the World Trade Centre in New York, and Briton Richard Reid was arrested in December 2001 for trying to blow up a plane headed to the United States.

“This liquid explosive type of attack is particularly worrying. Planes remain vulnerable and in the coming weeks terrorists will be thinking of something else to do that we have no idea about,” Peter Neumann, director of the Centre for Defence Studies at London’s King’s College university, told Reuters.

“If this had worked out it would have been beyond any doubt the biggest terrorist plot in the UK.”

<B>Flights cancelled</B>

The British Airports Authority BAA said it had asked all European carriers to suspend flights to London’s main Heathrow airport, where tighter security measures caused severe delays.

And British Airways said it had cancelled all short-haul flights to and from Heathrow for Thursday and some short-haul flights in and out of London’s second airport Gatwick.

Global travel information company OAG said more than 3,800 flights were due to take off from Britain on Thursday, with 11 percent bound for US airports. BAA said 180,000 passengers pass through Heathrow each day during the peak summer period.

Shares in European airlines fell on the news, with British Airways shares down more than 5 percent. The pound fell against the dollar and the euro. Oil prices fell towards $75 a barrel.

Airlines said no hand baggage would be let on planes leaving British airports, banning electrical or battery powered items. Passengers had to hand over all liquids, including contact lens solutions, on flights to the United States.

Neumann said the plot appeared similar to a 1995 plan to blow up 11 planes using nitroglycerine mixed in contact lens solution and a battery powered detonator.

Britain has been criticised by Islamist militants for its military campaigns in Iraq and Afghanistan.

Prime Minister Tony Blair has come under fire for following the US lead and refusing to call for an immediate ceasefire in the war between Israel and Lebanese Hizbollah guerrillas.

Blair’s office said the prime minister, who is on holiday in the Caribbean, had briefed US President George W. Bush on the operation during the night.

Independent terrorism expert Paul Beaver said the nature of the suspected plot also suggested a connection to al Qaeda.

“In the last two months al Qaeda promised that it would avenge Iraq and Afghanistan by attacking British and American aviation assets – I see a direct link with that,” he said.