The Jubilee Line is to close for five days between Christmas and the New Year as longer trains are introduced.
The line is due to reopen on New Year's Eve
An extra seventh carriage is to be added to all trains, boosting passenger capacity by 17%, and four new trains will increase the fleet to 63.
As six and seven carriage trains cannot run at the same time there will be a reduced service from now until December 25 when the line closes completely.
The line is due to be back up and running on New Year's Eve.
London mayor Ken Livingstone said: "This extra seventh car will be a real boost... increasing capacity by nearly a fifth across the whole line, or an extra 3,000 passengers every morning and evening peak."
He said the Jubilee Line will also receive a full line upgrade to increase capacity by 45% in 2009.
George McInulty from London Underground said the reduced service was "to help us ensure a smooth, safe and reliable delivery" of the seven-carriage service.
A replacement bus service will be in operation between December 26-30.