Carol singers perform in the Square over Christmas

The tree, a gift to Londoners from the people of Norway, was lit up in a ceremony attended by the Mayor of Oslo, Per Ditlev-Simonsen on Tuesday.

The Mayor of London said: "The Trafalgar Square Christmas tree creates one of the most magical and iconic images of this city."

A free programme of carol singing runs from 4 December to Christmas Eve.

Mayor Ken Livingstone said: "The tree is an important part of Christmas in London and the switching on of the lights signifies the start of the festive season.

"Londoners who sing around the Christmas tree are contributing to a great tradition, as well as being able to raise money for good causes."