London’s population overspill could be absorbed by 215,000 new homes built in Canterbury, Medway, Dover and Thanet, Boris Johnson is being advised.

This staggering figure, revealed in a report for the Mayor of London, would be on top of the new homes targets already set out in local plans.

It has prompted a warning from Canterbury council’s chief executive that substantial development is inevitable – and that those suggesting otherwise are in “Cloud Cuckoo Land”.

Colin Carmichael said government policy favoured new homes being built, and that local authorities had to toe the line.

He said: “It sounds like a doomsday scenario and I’m not saying I like it, but this is national guidance.

“The policy presumes in favour of development. If we try to say no, it’s like a rollercoaster. We just get rolled over”- Colin Carmichael, Canterbury council’s chief executive

“The policy presumes in favour of development. If we try to say no, it’s like a rollercoaster. We just get rolled over.”

Canterbury’s Local Plan already proposes the construction of 15,550 new homes in and around the city, including in Herne Bay and Whitstable.

But in a report supplementing London’s own local plan, Mr Johnson’s advisers suggest east Kent is ripe for far denser development.

Seeking commutable areas outside the capital where housing is currently at low density, they calculate Canterbury, Dover and Thanet could take 115,000 extra homes for Londoners, while Medway could accommodate an extra 100,000.

Even shared with east Kent’s other local authorities, Mr Johnson’s new homes would see Canterbury’s housing stock swell by 50% when added to its existing extra housing targets.

Though Mr Johnson’s figures are not binding, it is feared they will prompt the government to exert greater pressure on local authorities to stretch their housing targets even further.

Mr Carmichael said: “There are reasons to say no – it’s an Area of Outstanding Beauty, it’s on a flood plain, the transport links can’t cope – but if you don’t have a reason to say no, it’s difficult.”

He added that local authorities risk losing their new homes bonuses if they resist.

“If you say no to new housing and the developer takes it to appeal and wins, then you don’t get it,” he said.

“It will affect us all. This is national guidance. It’s cloud-cuckoo-land to suggest we can control this.”

Representatives of the city council, including Mr Carmichael, were due to meet representatives of the Greater London Authority to discuss the issue later this month.

Council leader John Gilbey said: “London is underperforming in terms of its housing targets and it’s seeing a way of dumping its excess population on us.

“Where are all these extra houses going to go?

“London is trampling on us. We have to fight it.”

Sir Edward Lister, Deputy Mayor for Planning said: “The Mayor is not asking Canterbury City Council or any local authority outside the capital to take London’s housing need as our populations grow.

"He is working collaboratively with councils to ensure London and the south east’s housing needs are met over the coming years.

"The Mayor recently set ambitious house-building targets that would deliver the largest number of new homes in London since the 1930s.

"This could be increased further through additional town centre and opportunity area intensification and utilising surplus industrial land with good transport access within London’s boundaries.”