British police said yesterday that they had foiled a plot to blow up aircraft mid-flight between the United Kingdom and the United States.

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

US Homeland Security Secretary Michael Chertoff told a news conference in Washington it could be an al-Qaida plot, but as investigations were still under way, it was not a definitive conclusion.

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

Last month, al-Qaida 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.

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 up 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 co-operating with foreign agencies.

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

Authorities declined to give the nationalities of those arrested but a police source said some were British.

Arrests were made in London, the southeast of England and Britain's second biggest city Birmingham.

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 long delays.

British Airways said it had cancelled all short-haul flights to and from Heathrow for yesterday 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 yesterday, with 11 per cent bound for US airports. BAA said 180,000 passengers pass through Heathrow each day during the peak summer period.

Source: China Daily/Agencies