A major terrorist plot to allegedly blow up aircraft in mid-flight was thwarted in a joint intelligence-led operation by the Metropolitan Police's anti-terrorist branch and security service, police said today.

It is believed that the aim was to detonate explosive devices smuggled on board the aircraft in hand luggage.

It is believed that the attacks would have been particularly targeted at flights from the UK to the USA.

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

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

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A Department of Transport spokesman said: "With immediate effect, the following arrangements apply to all passengers starting their journey at a UK airport and to those transferring between flights at a UK airport."

All cabin baggage must be processed as hold baggage and carried in the hold of passenger aircraft departing UK airports.

Passengers may take through the airport security search point, in a single (ideally transparent) plastic carrier bag, only the following items. Nothing may be carried in pockets:

* Pocket size wallets and pocket size purses plus contents (for example money, credit cards, identity cards etc (not handbags));

* Travel documents essential for the journey (for example passports and travel tickets);

* Prescription medicines and medical items sufficient and essential for the flight (eg, diabetic kit), except in liquid form unless verified as authentic;

* Spectacles and sunglasses, without cases;

* Contact lens holders, without bottles of solution;

* For those travelling with an infant: baby food, milk (the contents of each bottle must be tasted by the accompanying passenger) and sanitary items sufficient and essential for the flight (nappies, wipes, creams and nappy disposal bags);

* Female sanitary items sufficient and essential for the flight, if unboxed (eg tampons, pads, towels and wipes);

* Tissues (unboxed) and/or handkerchiefs;

*Keys (but no electrical key fobs).

All passengers must be hand searched, and their footwear and all the items they are carrying must be X-ray screened.

Pushchairs and walking aids must be X-ray screened, and only airport-provided wheelchairs may pass through the screening point.

In addition, all passengers boarding flights to the USA and all the items they are carrying, including those acquired after the central screening point, must be subjected to a secondary search at the boarding gate. Any liquids discovered must be taken from the passenger.

There are no changes to current hold baggage security measures.

The spokesman added: "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.

"These additional security measures will make travel more difficult for passengers, particularly at such a busy time of the year. But they are necessary and will continue to keep flights from UK airports properly secure.

"We hope that these measures, which are being kept under review by the Government, will need to be in place for a limited period only. In light of the threat to aviation and the need to respond to it, we are asking the travelling public to be patient and understanding and to cooperate fully with airport security staff and the police.

"If passengers have any questions on their travel arrangements or security in place at airports they should contact their airline or carrier."

A Metropolitan Police spokesman said: "We would like to reassure the public that this operation was carried out with public safety uppermost in our minds. This is a major operation which inevitably will be lengthy and complex."

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