A British Airways (BA) flight carrying 231 people from London to New York has turned around mid-flight and returned to London's Heathrow airport after a mobile phone started ringing.

A BA spokeswoman says flight BA179 has returned to Heathrow as a precautionary measure.

"A mobile phone was located on board the aircraft which none of the passengers appear to own," she said.

Britain imposed severe restrictions on hand luggage on board aircraft - including a ban on all electronic equipment - on Thursday after police said they thwarted a plot to bomb US-bound jets in mid-flight.

The alleged plot involved smuggling liquid explosives disguised as drinks or medicine and detonating them with a mobile phone or other electronic device.

The BA spokeswoman said the captain assessed the situation with the BA security team at Heathrow Airport, and although it was decided that it was safe to continue, the captain decided to return to Heathrow as a precaution.

"The captain explained his decision to the 217 passengers on board the aircraft," she said.

The aircraft will be security-searched at Heathrow before it departs again for New York this evening.

The spokeswoman said 12 cabin crew and two flight crew were also on board.

"We apologise to customers for the inconvenience but their safety is our number 1 priority and we will always err on the side of caution," she said.

UK Reduces Threat Level

Earlier, Britain reduced its security threat level from "critical" to the "severe" level, where it has been for most of the past year.

The Government raised the threat level to "critical" - the highest of five levels - on Thursday before arresting 24 people who were allegedly plotting to blow up US-bound passenger jets.

The Department for Transport's website says following the downgrade of the threat level, passengers will be allowed to carry one item of hand baggage through the airport security search point.

But the ban on liquids remains, with the few exceptions being prescription medicines in liquid form as long as they are verified as authentic, and baby milk and liquid baby food, so long as it is tasted by the accompanying passenger.

The statement says laptop computers will be allowed into the cabin of aircraft, but must be removed from the bag and placed in a tray when being X-ray screened.

Home Secretary John Reid says there has now been time to assess intelligence after the police operation.

"The police believe that the main suspects in the alleged plot were arrested last week," he said.

"The public needs to know that there may be other people out there who may be planning an attack against the UK. That is why there are a number of other security service operations under way."

'Very Serious' Threat Remains

Dr Reid says the change in the threat level does not mean that the threat has gone away.

"There is still a very serious threat of an attack," he said.

"The threat level is at severe, indicating the high likelihood of an attempted terrorist attack at some stage, and I urge the public to remain vigilant."

Dr Reid and Transport Secretary Douglas Alexander are scheduled to make a joint statement at 4:30pm AEST today.

The last time the threat level was set at "severe" was in the wake of the July 7, 2005 attacks on London's transport network.

The "critical" alert status is designed to show that the country faces the threat of an imminent attack, whereas the "severe" alert status means an attack is highly likely.

'Critical' status triggers exceptional maximum protective security measures to meet specific threats and minimise vulnerability and risk.