David Cameron and Boris Johnson suspended their EU referendum rivalry to inject some star power into Zac Goldsmith's campaign to become the next Conservative Mayor of London. The top Tory duo whipped up an already excited crowd in Goldsmith's constituency of Richmond, on the eve of the 5 May election.

The prime minister paid tribute to Johnson, whom he described as an "outstanding" mayor, and warned Londoners would become "lab rats" under Labour's Sadiq Khan.

"You need an outstanding candidate - that is what we've got with Zac Goldsmith. He's the right man for the job, you know that - he delivers for people, he works for people. He's not in it for the power or the glory, he's in it because he delivers for local people. That's what he will do for London," Cameron declared.

"Right now the last thing Londoners need is a tax rise. Zac has said he's going to freeze the council tax, what a contrast to his opponent who would put up council tax every year."

He added: "This isn't a referendum on one candidate. This is a choice. You don't need to know everything about Labour's candidate, you just need to know one fact; he nominated Jeremy Corbyn to be leader of the Labour Party and he doesn't regret that choice.

"If you want to be lab rats in Labour's experiment in London, then you go for the other guy. If you want someone who's a great candidate, a great man, a man with the right plan, vote for Zac Goldsmith."

With Goldsmith consistently behind Khan in the opinion polls and the latest survey from Opinium giving the Labour candidate a 14 point lead, Cameron drew on his surprise victory at the General Election to make light of the situation.

"Now you read the opinion polls, I think I'm someone who can tell you a few things about opinion polls, there is only one poll that matters - that is the poll on Thursday," he said.

Boris Johnson, as bombastic as ever, spoke after Cameron. The outgoing Mayor of London also attempted to link Khan with Corbyn, billing Goldsmith as the only candidate who can stop the "Corbynistas" from taking back City Hall for Labour.

"What did those [Ken] Livingstonians, those Corbynistas do? They tried to get back in 2012, obviously we repelled them again. But what did they do? They took over something, anybody know what they took over? The Labour Party," Johnson told the Conservative audience.

"The terrible truth is that the Labour candidate for mayor, Sadiq Khan, has emerged from the Corbynistas, the Livingstonians, he comes from that strain of political thinking. Do we want those types of people back in City Hall? There is only one man who can conceivably stop London lapsing back into the grip of the Labour Party and that man is Zac Goldsmith."

Johnson finished his speech with a rally cry to the Tory faithful: "We have 48 hours to keep out the high taxing, high spending Corbynistas. Do we want our city to be the laboratory, the Petri dish of Corbynism?"

Khan v Goldsmith: Who will win the brawl for City Hall? IBTimes UK