Campaigners fear plans to build new homes in a narrow country lane will cause chaos in an area already blighted by traffic.
The derelict Anmer Lodge daycare centre, in Stanmore, could make way for a Marks & Spencer and a 120 new homes if planning permission is approved.
But although neighbours are not against the idea of the new supermarket, they fear the extra homes will “overpopulate” the area.
Conservative Councillor Marilyn Ashton, who represents the Stanmore Park ward, said: “I am for a Marks & Spencer - but with the new dwellings, it’s all going to be too much and overwhelming.
“There’s going to be a lot of activity on a very restrictive site. It’s a very lovely road but a very narrow country lane - so it’s the wrong location for it.
“Traffic in the area is bad enough as it is and this will make it so much worse. It’s going to disturb people - there’s no question about it.”
Plans also include a car park with spaces for 294 cars, eight motorcycles and 144 bicycles.
The contract with Notting Hill Housing Trust was drawn up under Labour and exchanged under the Independent Labour administration, which commits the council to the development.
Cllr Ashton added: “We begged the Labour group not to sign those contracts, but the die has been cast now. I am sad and frustrated by it.”
The site will be accessed by Dennis Lane, Coverdale Close and Rainsford Close and plans also include a multi-storey car park.
But many feel the Marks & Spencer would be a welcome addition to the town by attracting more people to the area.
Raj Degamia, chairman of Stanmore Chamber of Trade, said: “We’re all for this new shop, providing everything is done correctly.
“We’re already in talks with the council about how we can regenerate the area, so it’s definitely a positive step in the right direction.”
Developers Notting Hill Housing say they have made a comprehensive traffic assessment in order to reduce the impact of the development in Stanmore.
A statement from Harrow Council said: “This application has yet to come before the council’s planning committee.
“Members will obviously carefully consider a range of views and planning policies and will take into account issues including the scale of development and the impact of traffic on the area.”