Travel fares will go up to pay for Crossrail, London Mayor Ken Livingstone has said.
The increase will cover the cost of borrowing money to pay for the £16bn project, he told a news conference.
Mr Livingstone said the rise has yet to be determined and would depend on the level of interest rates "in two or three years".
"Fares are going up to pay for the cost of the borrowing to do the construction," he said.
The rail link will connect Maidenhead, Berkshire in the west to Essex, through the centre of London.
Mr Livingstone had previously said the funding package did not require a fare increase.
It was said the money required would come from the extra revenue generated by passenger increases.
Seven new stations in central London would be built
However, Mr Livingstone was not specific about the level of fare rises or which services would be affected.
"I can't tell you exactly what interest rates are going to be in 2016," he said.
"Now until we know, because we will still be borrowing in that year for the construction...it might actually be slightly less, it might be slightly more."
Mr Livingstone was announcing some early decisions for the construction and operation of Crossrail.
He estimated the project would create up to 14,000 jobs, mainly in the construction industry.
Services are expected to start running from 2017, including the south-east section down to Abbey Wood, which will open up the Thames Gateway.
The mayor said Oyster travelcards would be accepted on Crossrail services and a premium fare would be charged on the Heathrow extension.
He said: "With a capacity twice that of the Jubilee line, Crossrail will mean many more people will travel by public transport to the primary business centres of London - the City, West End and Canary Wharf."