Scotland Yard has said they have discovered a plot to blow up flights from the UK to the US and commit "mass murder on an unimaginable scale".

There was a plan to detonate hand luggage on as many as 10 aircrafts.

21 people were arrested, and they believe to have caught the main players.

There are huge delays at all UK airports.

The threat level in the UK is now critical, which is the highest it can possibley be.  Which means "an attack is expected imminently and indicates an extremely high level of threat to the UK".

Three US airlines believe to be targeted.

BBC security correspondent Gordon Corera said the plot was thought to have involved a series of "waves" of simultaneous attacks, targeting three planes each time.

He also said the plan "revolved around liquids of some kind".

"Officials say the explosives would have been sophisticated and extremely effective," our correspondent said.

Metropolitan Police Deputy Commissioner Paul Stephenson said the alleged plotters had intended "mass murder on an unimaginable scale".

"We are confident that we have disrupted a plan by terrorists to cause  untold death and destruction and to commit, quite frankly, mass murder," he said.

"We believe that the terrorists' aim was to smuggle explosives on to aeroplanes in hand luggage and to detonate these in flight. We also believe that the intended targets were flights from the United Kingdom to the United States of America."

The terroists are British Born, but are believed to have links with Pakistan.

Currently no one is allowed to take hand luggage on any flights.  Only the bare essentials.

*All Information from BBC News*