Two Air New Zealand flights have taken off from London's Heathrow Airport this morning, as flights resume following interruptions caused by heavy snow across Britain.

Thousands of passengers around the world have been left stranded before Christmas after the airport was closed on Sunday and flights were cancelled as London was hit by some of the heaviest snow in decades.

Limited flights have resumed today as efforts are made to tackle the backlog of flights.

NZ38 to Auckland via Hong Kong, and NZ1 to Auckland via Los Angeles, both took off from London this morning, Air New Zealand spokeswoman Haley Mc Crystal said.

The flights, NZ38 a Boeing 777 and NZ1 a Boeing 747, had been "fairly full" of passengers because of Christmas, a spokesperson said yesterday.

Other flights from New Zealand to London have been cancelled.

Air New Zealand flights NZ39 and NZ2 are still scheduled to leave for London tonight, however if weather conditions do not improve they may get no further than Hong Kong and Los Angeles respectively.

"Therefore, we cannot guarantee that you will be able to reach your final destination," Air New Zealand said in a statement.

The airline warned there was limited accommodation in Hong Kong and Los Angeles.

Air New Zealand said it was continuing flights into Asia, North America and Canada tonight, but that if passengers had connecting flights to the UK or Europe on other carriers it was likely those services would be affected.

The airline has promised that flyers who decide not to travel will be entitled to rebook on another flight or claim a refund.

Meanwhile, international hubs including Paris, Frankfurt, Amsterdam and Brussels struggled to clear a backlog of passengers stranded over the weekend as holidaymakers tried to reach their destinations in time for December 25.

Frankfurt airport even resorted to sending in clowns to ease the frustration.

Heathrow yesterday warned travellers to anticipate delays and cancellations "potentially beyond Christmas day" as it fought to sort out its schedule.

The airport's Terminal 3 was turned into a makeshift camp with exhausted passengers crashed out on temporary mattresses.

Temperatures reached a record low in Northern Ireland, hitting minus 17.6degC.

Britain's National Grid forecast a record demand for gas on Monday, while the Automobile Association breakdown service forecast the day would "break all records" for emergency call-outs.

Eurostar, which operates high-speed passenger trains linking London with Paris and Brussels, cancelled some services due to the snow and operated speed restrictions on trains that did run, nearly doubling some journey times.

- With NZPA