(BBC) - A terrorist plot to blow up planes in mid-flight from the UK to the US has been disrupted, Scotland Yard has said.
It is thought the plan was to detonate explosive devices smuggled on to as many as 10 aircraft in hand luggage, reports Trend.
Police were searching addresses after a total of 21 people were being kept in custody after 25 arrests in the London area and West Midlands.
High security is causing delays at all UK airports, and MI5 has raised the threat level to critical.
According to MI5's website, critical threat level - the highest - means "an attack is expected imminently and indicates an extremely high level of threat to the UK".
Metropolitan Police Deputy Assistant Commissioner Paul Stephenson said the alleged plotters had intended "mass murder on an unimaginable scale".
Home Secretary John Reid confirmed that there had apparently been a plot "to bring down a number of aircraft through mid-flight explosions causing a considerable loss of life".
According to BBC sources the "principal characters" suspected of being involved in the plot were British-born. Arrests were made in High Wycombe, London and Birmingham.
In other major developments:
The US Department of Homeland Security increased the threat level applied to US-bound commercial flights originating in the UK
The Home Office confirmed there had been three meetings overnight and on Thursday morning of the Cabinet's emergency committee, Cobra, chaired by Mr Reid, to discuss the terror alert
A spokesman for Number 10 said Tony Blair had briefed US President George Bush on the situation during the night
All passengers were banned from taking hand luggage onto flights, while the government warned of delays.
BBC home affairs correspondent Daniel Sandford said he did not think the police believed an attack was imminent.
"The reason for raising the threat level is in case there is some other sub-plot, back-up plot around this that the police aren't aware of," he said.
Scotland Yard said in a statement that their investigation into the alleged plot was a "major operation" which would be "lengthy and complex".
"We would like to reassure the public that this operation was carried out with public safety uppermost in our minds."
BBC security correspondent Gordon Correra said given that most liquids were being banned from flights it suggested the suspected plot involved "some kind of liquid explosive".
He also said the plot had an "international dimension", while the investigation had involved the CIA.
"The Americans have definitely been involved for quite some time," he said.
Prime Minister Tony Blair is on holiday in the Caribbean, but Downing Street said the police operation was undertaken with his full support and he had been "in constant touch".