And he says that Boris Johnson, the current Mayor of London, could become Prime Minister after Mr Cameron steps down, saying he “could go all the way to the top of course”.

Mr Goldsmith also warns that a victory for Mr Khan would be a “massive thumbs up to the Corbyn project” which would make the Labour leader “immovable”.

However, it is his comments about Mr Khan’s links to numerous extremists that will prove most controversial and cause fury among Labour ranks.

“I think that we are in the middle now of an ideological battle, and it’s the biggest one we’ve faced in any of our lifetimes,” Mr Goldsmith says.

“And probably it’s fair to say we’re not winning, we’ve got great security services keeping us safe, but the problem is not shrinking, it’s growing. One of the things we need to do is isolate those people who have extremist views and come down upon them without making the rest of the community feeling isolated.