Air passengers travelling from Birmingham International Airport are facing major delays after a major terrorist plot to blow up aircraft in mid-flight was thwarted by the Metropolitan Police's anti-terrorist branch and the security service, police have said.

A spokesman for the airport urged passengers to turn up for flights as normal, but to help speed up the check-in process by carrying no hand luggage unless it was absolutely essential, and putting these items in plastic bags.

They were also asked to arrive at the airport in plenty of time and not expect to check in at the last minute.

A spokeswoman said: "We are operating to usual schedules. There are a few minor delays, some up to an hour, but they are moving through. We are not grounding flights but passengers should expect some delays."

A number of people were arrested in London overnight - the culmination of a major covert counter-terrorist operation lasting several months.

Home Secretary John Reid said police had carried out "a major counter-terrorism operation to disrupt what we believe to be a major threat to the UK and international partners".

He said the alleged plot was "very significant" and was designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life".

The Home Secretary warned that the operation would mean major disruption at all UK airports from today but he added: "As far as is possible we want people to go about their business as normal."

Following the police action, security at all UK airports has been increased and additional security measures been put in place for all flights.

All hand baggage has been banned from aircraft cabins, other than essential items such as a small wallet or purse, travel documents, sanitary products and basic provisions for infants. Parents were being asked to taste baby milk before boarding.