Boris Johnson has told HS2 campaigners in Ruislip if he is elected next year he will oppose the rail scheme unless certain conditions are met.
He listened to their concerns about a lack of detail from HS2 on the effect of work due to take place before and during construction of the line - in particular the impact on West End Road near to Ruislip Gardens - where many of his prospective constituents will be affected.
Mr Johnson said: "I can reaffirm my opposition to this scheme unless we can be satisfied beyond doubt that any proposed mitigation builds upon the success we have already achieved by having the route tunnelled through Ruislip.
"Defeat the need for the Heathrow spur and the tunnel can then go beyond Harefield. My support is also conditional on a suitable direct link to HS1," he said.
Lottie Jones of Ruislip Against HS2 said the group was grateful for the opportunity to meet with Mr Johnson to discuss HS2 directly in more detail.
"It was a useful meeting and we were able to draw his attention to many of the ongoing local concerns," she said.
"Despite the route being tunnelled through what would be his potential constituency there are still enormous impacts from construction work and subsequent blight.
"Boris shares our concerns for many of the wider impacts of HS2 on the borough and the importance of extending the tunnel to protect the homes and landscape of Ickenham, Harefield and in particular, Hillingdon Outdoor Activities Centre ."
In April, Mr Johnson handed a petition to parliament calling for the tunnel to be extended.
He has also been vocal about his opposition to the Heathrow spur - a track which will connect the HS2 line at west Ruislip to Heathrow Airport - calling it a "ludicrous waste of money."
The decision on the Heathrow spur was put on hold until after the Davies Commission report on airport expansion is published next year, but a 'passive provision' for building the spurs - the junctions where the tracks will connect to the HS2 line - is the reason requests to extend the Ruislip tunnel to beyond Harefield are unlikely to be agreed.
This is because connections between the spurs and the main line HS2 track could not be sited in tunnels, they would have to connect above ground.
Mrs Jones said she has urged the Mayor to lend his support to the campaign by local MPs Nick Hurd , Sir John Randall and Dominic Grieve to get Transport Secretary Patrick Mcloughlin to officially scrap the Heathrow section of the HS2 route.
"We feel sure Boris understands how important an issue HS2 is to our community and this will not be lost on him as we head towards the election next year," she said.