Dr Onkar Sahota AM accused the Mayor of London of “washing his hands” of the problem when he was dissatisfied with the response given by him at a Mayor’s Question Time meeting.
Hillingdon Hopsital recently announced it could not go ahead with a "much-needed" temporary car park due to costs.
Dr Sahota said: "As Hillingdon Hospital faces increasing pressures resulting from the biggest programme of hospital cuts and closures north-west London has ever seen, demand for the limited and costly parking spaces at the hospital will only increase.
“When I urged the mayor to take a stand on this important issue, he simply batted the ball into the court of the NHS, refusing to criticise the decision. Quite why the mayor believes his constituents should continue to have such poor access to the hospital is not clear.”
He said: “I am aware of local concerns around aspects of the current redevelopment, including car parking provision. In the most recent meeting earlier this month I sought and received assurances over car parking provision.
“I have seen nothing from the trust to suggest that their planned addition to the current car park won’t be built.”
A spokesman for Hillingdon Hospital say they too are considering options, after the costs of building a temporary car park were deemed too high.
He said: “The trust continues to recognise that parking capacity does not adequately meet the levels of demand being experienced and will continue to consider options to improve parking capacity at the Hillingdon Hospital site, as a matter of urgency."
Dr Onkar suggests the mayor's dismissal of the matter was part of his “war against motorists in outer London”, which has seen him “reject 50% of major planning applications in the borough because they provide too many car parking spaces”.
The Labour AM added: “Maybe the mayor has just failed to grasp how patchy public transport provision is in outer London, in many places creaking under immense demand. Residents will be forgiven for expecting their mayor to share and act on their concerns about the local hospital.
“Instead they are left wondering whose interests he actually represents.”
Mr Johnson, in response to Dr Onkar's claims, pointed out that Hillingdon Hospital has seen £15m of “much-needed investment” in a brand-new acute medical unit, a state-of-the-art endoscopy unit, significant refurbishments in maternity services and improvements in the rehabilitation ward for elderly patients.
He added: “Unfortunately many of the hospital buildings remain in very poor condition and will require substantial investment in the future.
“As mayor and as the local MP, I am working closely with the trust and the other local MPs to identify how best to improve hospital provision for local people, including plans to rebuild the entire hospital.”