Sadiq Khan has been warned it will be “impossible” to pedestrianise Oxford Street without dramatic changes to London’s bus routes.
The Mayor has pledged Britain’s busiest shopping street would be traffic free by 2020.
But the London Assembly’s Transport Committee warned that a “sizeable reduction” in the number of buses using the road was necessary.
In a letter to Mr Khan, they wrote: “It is impossible to conceive of a pedestrianised Oxford Street until this happens.”
The pedestrianisation of Oxford Street was a key pledge of Mr Khan's manifesto during his successful bid to become mayor.
The committee welcomed Mr Khan's "bold ambition", stating the road's accident rate and air pollution levels have "serious human costs" and a negative impact on London's economy.
But they warned that simply diverting buses into surrounding streets "should be avoided" due to the effect on nearby communities.
A consultation of local stakeholders was welcomed by London Chamber of Commerce and Industry policy manager Siwan Puw, who said it was important that businesses near Oxford Street "do not suffer unnecessarily as an unintended consequence" of pedestrianisation.
She added: "While it is important that congestion on Oxford Street is improved and largely inevitable that it is pedestrianised, it is crucial that it is done so with a proper plan and shared vision."
Responding to the letter, a spokesman for the mayor said: "The pedestrianisation of Oxford Street will make this world-famous street safer, and improve the air quality for the thousands of people who use it every day.
"The mayor recognises that this will be a complex process and has already begun to discuss with stakeholders how to best to make this vision a reality.
"He looks forward to continuing to engage with local businesses, residents, road users and Westminster City Council as this exciting scheme to create the best outdoor shopping experience in the world develops."
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