Conservative City Hall hopeful Zac Goldsmith has risked trade union fury with a plan to boost police numbers on the London Underground. The top Tory has promised to primarily pay for the 500 extra British Transport Police officers by 'reviewing' free travel passes for the friends and family of Transport for London (TfL) workers.

Even lodgers and flatmates of the staffers are eligible for the 'nominee passes', and the initiative means the network loses out on £22m ($31m, €27m) per year, according to a 2015 report from Conservatives on the London Assembly . But the figure has been disputed by TfL, who claimed scrapping the scheme would only boost revenues by £7m per year.

Goldsmith argued that axing the programme would save "at least tens of millions" when he addressed reporters at his crime manifesto launch in Westminster on 6 April. "I think the money would be better spent having 500 additional British Transport Police on our tube network," the Conservative MP added.

"We know that the threat has grown and if anybody disputes that we know that anxiety levels have gone through the roof when it comes to the threat of terror, particularly in this part of the London transport system."

Zac Goldsmith crime manifesto
Goldsmith unveiled his crime manifesto with the help of Home Secretary Theresa May in Westminster IBTimes UK

The proposal has infuriated trade unions linked with TfL. Mick Cash, the general secretary of the RMT, branded Goldsmith's costings a "total joke".

"At a time when the multi-billion pound tax-dodging racket of the super-rich is all over the media it is disgusting that Tory multi-millionaire Zac Goldsmith is threatening the long-established travel concessions for hard-working London transport workers," Cash declared.

"Not only are the hereditary multi-millionaire Goldsmith's figures a total joke but the fact is that if London needs more investment in transport and policing it should come from the gangsters, crooks and tax-dodgers who use this City as a dumping ground for their dirty money."

Manuel Cortes, the general secretary of the TSSA, also blasted the plan. "Zac Goldsmith is a playboy trying to play in politics. But London is not a playground. How dare this trust fund Tory think tube workers can be kicked around like a political football," the union chief said.

The comments come after Home Secretary Theresa May gave a personal endorsement of Goldsmith at the manifesto event. "We need a Mayor who can work with this Conservative government to carry on reducing crime, protecting victims and helping to prevent terrorism on our streets and extremism in our communities," the cabinet minister said.

Labour's Sadiq Khan, the front runner in the Mayor of London election, used the government's and Boris Johnson's record to attack Goldsmith. "The last Tory Mayor broke his election promises to cut crime and improve confidence in the police," the Tooting MP argued.

"And the number of police officers in London is down 1,535 since the Tories came to power. It would be exactly the same under Zac Goldsmith – fewer police officers, more cuts and Londoners left less safe."