There has been a 91% rise in cyclists across London, figures show
Cycling campaigners in London said money given to boroughs for cycle safety improvements is to be cut.
The London Cycling Campaign claimed funding had been slashed in half, from £20m to £10m in 2008/9.
The Green Party said the cut would represent a complete turn around of the Mayor's pledge to improve cycle lanes.
But the Mayor's office said money for cycling would still go up overall and would not be used, as some had claimed, just to rephase traffic lights.
The London Cycling Campaign said hundreds of planned cycle safety improvements to junctions and local streets were now under threat.
The organisation is calling on the Mayor to reverse the cuts and consult with cycling groups on completing the 500-mile London Cycle Network.
"Slashing the funding for cycle routes means they will be left unfinished.
"This will undermine the Mayor's flagship programmes such as the mass cycle hire scheme because people won't feel safe on the roads," said Koy Thomson, the campaign's Chief Executive.
Many of the cycle schemes are also part of programmes for general improvements to town centres.