Michael Portillo has said he won't be voting for Boris Johnson in the London mayoral elections.

Speaking on BBC's This Week on Thursday evening, the former Conservative defence secretary and one time leadership candidate refused to endorse his party's candidate for mayor.

Portillo said he would support "a candidate who endorses a third runway for Heathrow airport because I think that is fundamentally important to the capital and I can’t understand any candidate presenting himself or herself without such a commitment”.

Pressed to confirm that he wouldn't be supporting Johnson, Portillo replied simply - "Correct".

All the mayoral candidates have expressed their opposition to a third runway for Heathrow airport, except for independent candidate Siobhan Benita.

Portillo is the first senior Conservative to declare they aren't backing Boris Johnson's re-election bid. His announcement comes after Labour peer Lord Sugar said on Twitter that Londoners should not vote for Labour candidate Ken Livingstone.

"I don’t care if Ed Miliband is backing Livingstone. I seriously suggest NO ONE votes for Livingstone in the Mayoral elections," he wrote on Twitter.