The number of bus journeys taken in London fell in 2012 for the first time in 15 years, according to figures released by the Department for Transport.
More than two billion bus journeys were made in the city last year which was a drop of 9.3 million compared with the previous year.
The Green Party said the fall was due to a hike in bus fares.
However Transport for London (TfL) said the fall was due to the Olympic Games.
TfL's surface transport managing director Leon Daniels told BBC News: "Last year it was the Olympic Games, so during the course of 2012 on all sorts of days across the summer, we had lots of our streets closed and here at TfL we were asking people to travel to work different ways."
Figures showed 2.3 billion bus journeys were made in London last year which was a drop of 0.4%.
It was the first time the number of bus journeys has fallen since 1998.
TfL said the colder than average temperatures last year and the timing of Easter had also affected people using buses across the city.
Passenger numbers were overall on an upward trend and are expected to rise for the next financial year.
Bus fares rose by five pence at the beginning of this year from £1.35 to £1.40, or if passengers pay by cash it is an extra £1.00.
Darren Johnson, London Assembly Green Party member, said bus fares should be cut not raised.
"The cost of living is going up in London and more people really are being squeezed. These fare increases are putting people off using public transport."
Labour MPs in London have signed a Commons motion calling for transport fares in the capital to be kept to the rate of inflation for 2014.
The motion signed by 24 MPs said fares in London were "the most expensive in the world" and since 2008 the cost of a single bus journey has jumped by 50%.
A spokesperson for London Mayor Boris Johnson, said earlier this month: "Every penny from fares is invested in improving transport for Londoners, resulting in one of the biggest and most reliable transport systems in the world. "