A major terrorist plot to blow up an aircraft in mid-flight in the United Kingdom has been foiled and a number of people have been arrested, police said on Thursday.

British Intelligence agency M15 said the current threat level is critical -- the highest possible -- and security at all airports in the UK has been increased with additional security measures put in place for all flights.

It is thought that the plan was to detonate explosive devices smuggled onto aircraft hand baggage and attacks would have particularly targeted flights from the UK to the United States, Scotland Yard said in a statement.

Only bare essentials -- including passports and wallets -- will be allowed to be carried on board in transparent plastic bags. However, there are no changes to current hold baggage security measures.

Overnight, police arrested a number of people in London - the culmination of a major covert counter-terrorist operation lasting several months.

"Regrettably, significant delays at airports are inevitable. Passengers are being asked to allow themselves plenty of extra time and to ensure that other than the few permitted items listed above, all their belongings are placed in their hold baggage and checked in," a Metropolitan Police spokesman said.