Hosting the start of the Tour de France will bring £70m to the capital in 2007, Transport for London (TfL) has said.
American Floyd Landis won this year's Tour de France
The prestigious annual three-week cycle race will begin in London next July, after TfL paid £1.5m to host it.
It predicts a boom for the capital's hotel industry during three days of opening events in London.
Transport commissioner Peter Hendy said: "Considering the amount of money we put in, we are getting a lot back over a short period of time."
'Good for UK'
This year's Tour de France was won by American Floyd Landis as the gruelling 3,657km race finished in Paris on Sunday.
Mr Hendy said: "London is going to be ready for this. It is the sort of spectacular that will be good for the UK."
TfL "conservatively" estimated it would get a £70m return on the £1.5m it paid organisers Amaury Sport Organisation to host the start.
Another £5m has provisionally been allocated to cover running costs.
It expected 120,000 people would stay in hotels and bed and breakfast accommodation during the opening weekend.
The 2007 opening ceremony will take place in Trafalgar Square on 6 July, with the first stage of the race going through London and Kent on 8 July.
Bradley Wiggins, Britain's 2004 Olympics individual pursuit champion, said: "This is just like the Olympics coming to town for the day.
"It is a one-off and it is going to be enormous. It is really going to build up over the next year."