The cuts were revealed last week when the mayor announced £168 million in funding for London boroughs to deliver local transport improvements in 2009-10. Funding for the 800km London cycle network is down to £10 million from almost £20 million in 2008-09.
The London Cycling Campaign said the cuts undermine Johnson's plans to more than treble cycling levels in the capital. Hundreds of planned safety improvements to junctions and local streets are now under threat, it added.
The lobby group called on the mayor to reverse the cuts and consult with cycling groups on completing the network. Chief executive Koy Thomson said: "This will undermine flagship programmes such as the mass cycle hire scheme because people will not feel safe on the roads."
A mayoral spokesman said £55 million is being invested in cycling this year. "The cycle network has an important role and with other initiatives will make London a true cycling city," he said.