When Nick Clegg was told that Boris Johnson was no actually on the line, but had pre-recorded his message, the Deputy Prime Minister responded: "That's easy for him, isn't it?"
Boris, if you're listening - and I suspect he isn't - I think we have actually cut the amount of taxpayer money which is used to pay for the cars and everything for ministers by, I think, about 70%. We have massively slashed it and changed it.
Clearly, ministers - myself included - drive in cars, but not all the time. I am about to jump into a train to go up to Sheffield this afternoon.
I think the fact we have cut the bill we inherited from Labour for ministerial cars by, I think, around 70% is a big step in the right direction."
London Mayor Boris Johnson ambushed Nick Clegg on his radio phone-in, urging him to get his Cabinet colleagues "out of their posh limos" and on to public transport.
Mr Johnson recorded a message for LBC 97.3 radio to demand more spending on London's transport system.
Hi Nick, it's Boris here from Islington.
I just want to ask you when are you going to get all those Government ministers out of their posh limos and onto public transport like everybody else?
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 - rather than getting down on public transport with the rest of us?
Nick, get them out of their limos! Boris, over and out.
But Mr Clegg, appearing on his weekly phone-in show, insisted the coalition Government had cut spending on ministerial cars by around 70% and challenged the Conservative Mayor in his turn to make use of billions of pounds worth of Treasury guarantees for infrastructure projects.