Thousands of frustrated travellers remained stranded at airports across Europe on Tuesday, even as authorities warned that disruption could last until after Christmas, following the heaviest snowfalls in decades.

Meanwhile, most of the London-bound flights from India resumed on Tuesday as the Heathrow airport partially opened for operations after being hit by heavy snow for the past two days.

Thousands of Christmas travellers were forced to sleep in airports and train stations across Europe as fresh snowfalls paralysed transport system.

At Heathrow, Britain's busiest airport, hundreds are spending their fourth day stranded in terminal buildings which looked like Mumbai's busy Chhatrapati Shivaji Terminus.

Beleaguered BAA, the operator of Heathrow, has scrapped two thirds of Tuesday's schedule and one of the two runways is still closed, despite it not snowing heavily since Saturday morning.

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Air India operated two flights to India on Monday, one to New Delhi and the other to Mumbai, but no flights were allowed to take off to India today, a spokesperson of the airlines told PTI.

Barring a Kingfisher Airline flight from Delhi (IT-001), all other flights of Air India, British Airways, Virgin Atlantic, Jet Airways operated according to the revised schedule.

Kingfisher's flight from Mumbai (IT 007) left for London around 3 pm, while Jet's flight (9W 120) had a scheduled departure at 6.30 pm.

The airline has planned to operate its London-Delhi flight (IT 002) from Heathrow subject to availability of de-icing fluid at Brussels airport as its local supplies had been exhausted and fresh supplies were expected to take quite a while. Also availability of arrival slot clearance from Heathrow for the arriving aircraft, an airline spokesperson said.

From Delhi, Air India, British Airways, Virgin Atlantic and Jet Airways operated their flights but on a revised schedule.

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Flights to and from London have been affected for the past two days due to inclement weather condition in the United Kingdom.

London's Heathrow has been the worst-hit airport with two-thirds of flights being cancelled until Wednesday while other UK airports, including Gatwick, have also been badly affected due to snow. Gatwick airport reopened at 0600 GMT after remaining close overnight. One of the Heathrow's runway was closed on Monday.

Passengers have been warned to expect more delays and cancellations, possibly up to Christmas Day.

In Germany, over 300 flights were cancelled at Frankfurt yesterday after overnight snow, while operations at Paris' two main airports -- Charles de Gaulle and Orly -- also remained affected after weekend cancellations.

On Monday, all flights to London from the country were cancelled as the Heathrow airport remained closed.

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The airlines have asked the passengers to be in touch with their call centres or the travel agents. They are also allowing the passengers to cancel and rebook their ticket without extra charges.

Some of the passengers who have to travel in urgency to London are taking flights to Paris or other nearby destinations in order to reach London at the earliest.

The British Airways operated its London-Delhi flight yesterday, though it took off after the passengers spent four hours inside the plane at the Terminal 5.

"Passengers should anticipate further delays," a BAA spokesman said.

The operator warned "significant cancellations" would last until at least Wednesday and that many part of the airfield would be out of action until Wednesday morning "at the earliest".

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"The key message for our passengers is to make sure you check on the Heathrow website to see if your flight is going to depart, if it is, come to the airport and we will do our absolute best to give you a great experience," Colin Matthews, chief executive of British airport operator BAA, was quoted as saying by ITV news.

Most of the Europe-bound flights were cancelled while the British Airways operated their long-haul flights including those for Japan.

Temperatures hit a low of minus 17.6 degrees Celsius (0.3 Fahrenheit) in the northwestern town of Crosby, although in London they hovered around 0 degrees Celsius (32 Fahrenheit).

Forecasters are now warning that another 12 inches of snow could fall in some parts of Britain before Wednesday morning, with Wales and the south west likely to be worst hit.

The sub-zero temperatures is expected to last until Christmas but that the long-awaited thaw could begin on Boxing Day.

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Elsewhere, many roads were still treacherous due to continued snow and ice and there were huge queues for the Eurostar at St Pancras in London.

One enterprising German family became so fed up with Heathrow's cancellations they sent a taxi for their son -- all the way from Dusseldorf.

In Germany, more than 1,000 travellers spent the night at the country's main airport Frankfurt. On Monday, 376 flights out of 1,400 were scratched at Frankfurt as fresh snowfall caused gridlock.

French authorities allowed the two main airports in Paris, Charles de Gaulle and Orly, to remain open around the clock to clear the backlog of delayed flights.