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David Cameron has warned Londoners they would become the “lab rats” for Jeremy Corbyn’s “disastrous political experiment” if Labour’s Sadiq Khan is elected next Mayor of London.

The Prime Minister revealed the central weapon in the Conservative’s armoury as he launched the attack on the Labour candidate at a party rally with Tory hopeful Zac Goldsmith.

Tory campaign insiders have told the Standard they believe Mr Corbyn is Labour’s biggest liability in the race for City Hall.

The Prime Minister’s attack on the Labour candidate as “Corbyn’s man” at the event in Mill Hill last night came as latest opinion polls put Mr Khan ahead by seven points in the tightly fought contest.

Mr Cameron told party supporters: “Just think for a moment about what having the first Corbyn elected mayor in London would be like for our economy, for our education, for our politics, for our city. It would be a disaster.

“You don’t want to be lab rats in the first Corbyn economic experiment in public life in our country, so we’ve got to win this.”

Mr Goldsmith joined the attack saying: “If Sadiq Khan wins in May we will see London for four years run as a laboratory, a testing ground for Corbyn’s radical politics. That is a price we cannot pay.”

Mr Khan, who nominated Mr Corbyn for the leadership but did not later vote for him, has gone to great lengths to distance himself from the Labour leader.

Since being selected last summer, he has clashed with his party leadership on issues including high tax rates, airports and the abolition of the City of London Corporation.

He also criticised Mr Corbyn when the Labour leader opted to remain silent at a service to mark the 75th anniversary of the Battle of Britain.

When asked by the Standard if he was concerned at Labour’s lead in the polls despite the ongoing turbulence within Labour, Mr Cameron said: “It’s a very vital contest and it’s definitely a close election. We are taking nothing for granted.

“It is a choice between two candidates. Sadiq Khan backed Corbyn’s crazy economic plans and we don’t want to bring that to London.”

The Prime Minister portrayed Mr Goldsmith as the heir to Boris Johnson, who outperformed his own party in London, saying: “Zac is not a party man, he doesn’t do things that the party tell him. I know that from my own experience. He’s not in it for the prestige or the fame or the photocalls.

“Zac Goldsmith stands for the people he represents, nothing else and that’s what he will do for London. No party posturing, no funny games.. just simply standing up and delivering for London.”

Mr Goldmsith added: “I’ll get a good deal from Government and get a good deal for this city. Dust them up if you have to, that is a prerequisite for the job...

“I’m sure the Prime Minister, in a private room, would say I have been a pain in the backside, but I have delivered for my constituents and I’ll deliver for London.”

They also both spelled out the need for more police on the street and to tackle the threat of terrorism.

Mr Cameron said: “We need to recognise the scale of the terrorist threat we face in London. What happened in Paris could have happened here and I will do everything I can to help keep London safe. I know working with Zac I’m going to be able to do just that."

He said they would be "a mayor and a prime minister that know we have got to be tough on the Islamist extremists that want to divide our country.

“We know we have to be tough in terms of providing extra police on the streets and making sure there’s sufficient armed officers to respond if there is an incident and that will be the case with a Conservative mayor.”

Mr Khan tackled Mr Cameron’s attack head on, telling the Standard: “I’m shocked that the Prime Minister described Londoners as ‘rats’. I love this city and the people who live here. Desperate stuff from a desperate campaign.”

Labour MP Stephen Pound added: “Sadiq has said time and again that he’s his own man - he’s publicly disagreed with Jeremy on issues like Trident, airports and business.

“The Tories have no positive vision for London and this shows that their campaign is getting rather desperate.”