Deputy Prime Minister Nick Clegg got a surprise when Boris Johnson ambushed his radio phone-in show.

Mayor of London Johnson demanded Clegg force government ministers out of their "posh limos" and onto public transport during a live LBC radio phone-in.

Johnson said: "Hi Nick, it's Boris from Islington. When are you going to get all those government ministers out of their posh limos and onto public transport like everybody else.

"And how can we possibly expect government to vote for increases in infrastructure spending, which we need in this city, and upgrading the Tube which we all need, when they sit in their chauffeur-driven limousines paid for by the taxpayer.

"Nick - get them out of their limos. Boris, over and out."

When host Nick Ferrari informed Clegg that Johnson's question had been pre-recorded, Clegg responded in exasperation "well, that's easy for him."

Clegg insisted the coalition government had taken action on driving down the cost of the ministerial fleet of vehicles for senior politicians, which costs £82,000 a year per vehicle.

"We've cut the amount of money used to pay for the cars and everything by about 70 percent," said Clegg. "We've massively slashed it...[it's] a big step in the right direction."

The deputy prime minister also hit back at Johnson by challenging him on big projects such as homebuilding.

Johnson this week unveiled his vision for building "a million homes" for the capital city and demanded tax breaks from Chancellor George Osborne to make it happen.

Clegg said that the Treasury had offered £50bn in guarantees for infrastructure projects

"So my challenge to Boris is," he said, "is he going to use these guarantees to help Londoners?"

Clegg yesterday announced yesterday that the Liberal Democrats could drop calls for a "mansion tax" for wealthy households, in exchange for higher council tax rates on valuable properties worth more than £2m.

He insisted he was "proud" of raising the income tax floor, making the lowest paid "£6,00 a year better off since 2010."

Shadow cabinet secretary Rachel Reeve said Clegg was "desperate" with the economy continuing to struggle in recovery from recession.

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