A study did not provide any analysis of accident rates
Mayor Boris Johnson has been urged not to make a snap decision to allow motorbikes to use London's bus lanes.
Pedal cyclists, who already use the bus lanes, claim plans for the change were based on a study which was "not considered sufficiently reliable".
They are due to hand a 3,000-name petition to the mayor next week.
A spokesman for the mayor said there would be further consultation on the plan and different views would be taken into account.
Permitting motorbikes to use bus lanes is designed to cut accident rates.
A trial allowing motorcycles in bus lanes was carried out by Transport for London to see if it would reduce the number of accidents involving bikes, without impacting on other road users.
But the limitations of the study meant it did not yield any analysis of accident rates.
The mayor's spokesman said as a committed cyclist the mayor had an "obvious interest" in ensuring the roads were as safe for cyclists as possible.
He said previous reports had not predicted a significant increase in accidents to pedestrians or cyclists.
He added: “Further consultation must happen before motorcycles can legally use Transport for London's bus lanes."
London Assembly member Jenny Jones will present the petition to Mr Johnson on behalf of the London Cycling Campaign (LCC).
In a letter to the mayor, LCC's chief executive Koy Thomson said: "While we would support measures to make motorcycling safer, such as a 20mph speed limit, there is no clear environmental, safety or congestion reason for allowing motorcycles into bus lanes."
He added that "making a snap decision on this key issue based on evidence that is 'not statistically significant', or is based on 'flawed methodology', is an enormous risk".
Jeff Stone, of the British Motorcyclists Federation (BMF), said allowing motorcycles in bus lanes would make it safer "for all vulnerable road users".
He said many cyclists were also motorbike riders, but some in the cycling community were "scaremongering" believing they had "a devout right to use bus lanes and that nobody else should use them as well".