The southern runway at Heathrow Airport has reopened after four days, airport operator BAA has said.
Thousands of passengers have been stranded at the airport since heavy snowfall on Saturday grounded planes, with 700 flights cancelled on Tuesday.
Eurostar said it hopes to run a "near normal" service on Wednesday. Hundreds of people queued at St Pancras station overnight as services remain disrupted.
There are also delays on several train routes serving London.
A limited service is now operating on the East Coast route between Peterborough and London King's Cross, after it was suspended following severe damage to overhead power lines at Huntingdon.
Eurostar 'near normal'
Heathrow Airport has seen cancellations with backlogs piling up as the weather conditions forced thousands of passengers to camp at the west London airport.
A third of flights took off from the airport on Tuesday, and passengers have been advised only to travel to the airport if their flight has been confirmed to reduce congestion.
Although the airport has been able to clear snow from the docking bays for aircrafts, BAA's chief executive Colin Matthews warned it could be some time before normal operations resume.
Earlier Prime Minister David Cameron said: "Even BAA's harshest critics have conceded that given the amount of snow that has fallen, extensive disruption is understandable.
"But 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."
Earlier defending the airport's handling of the situation, Mr Matthews said more than 100 extra contractors had been brought in to help clear snow.
Up to 100 people queued at the Eurostar terminus overnight on Monday with hundreds more joining them later in the day.
A message on Eurostar's website said: "We are pleased to advise that Eurostar will revert back to checking-in passengers for the service that you are ticketed on.
"We ask that you come to the station only one hour prior to the departure time shown on your ticket, as we will board services throughout the day in strict rotation."
Eurostar warned of delays to the journeys and added it was cancelling nine of the total 52 services out of London.
Despite the reopening of Heathrow's southern runway thousands of people are still facing another night at the airport.
Mr Matthews said: "We have arranged more hotel rooms, marquees have been set up to provide additional shelter and staff had been taken out of the offices to provide assistance to passengers."
There are also delays at London City Airport due to the adverse weather conditions.
Forecasters have predicted fresh snow flurries in London overnight.