The second runway at Heathrow has reopened, but stranded passengers have been warned not to expect normal service to resume at once.

Thousands of people have endured uncomfortable nights in the airport's terminals as their travel plans suffer continued disruption due to the snowy weather.

David Cameron said "If it's understandable that Heathrow had to close briefly, I'm frustrated on behalf of all those affected that it's taking so long for the situation to improve.

"There have been intensive discussions between (Transport Secretary) Philip Hammond and BAA about how best to ensure that normal flying capacity is resumed as soon as possible."

Mr Hammond said the Government had offered the airport operator the use of troops but was told they were not needed. Heathrow's second runway reopened at 5pm after snow ploughs cleared it, prompting hopes of a let-up.

But BAA chief executive Colin Matthews warned people not to expect the situation to return to normal immediately just because the second runway has reopened.

He said: "It is good news to see aircraft taking off and landing from two runways but it's really important that passengers understand that doesn't mean the full schedule is going to be restored instantly.

"It's not just two runways we need - we need every other link in the chain to be fully up to speed and it's going to take some time to do that."

The airport plans to run about two-thirds of the schedule on Wednesday but it is "absolutely vital" passengers check first before going to Heathrow.

There were also flight disruptions at Aberdeen, Edinburgh and Glasgow, as well as Gatwick, London Luton and London City airports.

And hundreds of people were evacuated from stricken trains following damage to overhead power lines at Huntingdon, near Peterborough. The East Coast line, one of the country's main railway arteries, was suspended between London and Peterborough, although it reopened later with a limited service operating.