The Prime Minister has called on union leaders to call off a planned 48-hour strike on London Underground which is set to cause travel chaos in the capital over the next few days.
David Cameron's official spokesman warned of "misery" for passengers because of the walkout from 9pm tonight.
The spokesman said Mr Cameron "thinks that Bob Crow's strike is plain wrong and Bob Crow should call it off rather than inflict misery on hard-working families in London".
The comments followed a confrontation between London's mayor and Mr Crow, the Rail Maritime and Transport union (RMT) general secretary .
Mr Crow and Transport Salaried Staffs Association (TSSA) general secretary Manuel Cortes have accused Boris Johnson of refusing to meet them to discuss the closure of ticket offices, and the pair went to City Hall to try to confront him.
The mayor was at the central London studios of LBC radio for his weekly phone-in when Mr Crow spoke to him on his mobile phone, on-air from outside City Hall.
The RMT chief said: "We are not here to score points - all we want is an opportunity to negotiate about the Tube. We are asking you to listen to our point of view. We would love to call the strike off."
Mr Crow accused the mayor of refusing to suspend the ticket office closures.
Mr Johnson replied: "That is complete nonsense. We are more than happy to engage on these issues.
"Of course there are job losses involved but there are no compulsory redundancies. We have already had more than 1,000 people showing an interest in voluntary redundancy.
"Call off this pointless strike which will do nothing other than cost your members their wages."
Mr Crow said later it was clear the mayor was still refusing to meet unions.
Tube services will start to be disrupted later today after talks failed to resolve the row .
Members of the RMT and the TSSA will walk out for 48 hours from 9pm and again at the same time next week. Transport for London warned that services will be hit from this evening until Friday morning .
Mr Crow said: "We would love to call the strike off, this strike can be called off ... a form has been sent over saying that these jobs are going."
If that form was suspended, Mr Crow said, "we can suspend our industrial action" but, turning Mr Johnson's claim back on him, he accused the mayor of holding a gun to the heads of the unions.
"The problem that we have got, we have got the chief executive officer of London Underground coming to our negotiators saying he's got the authority from the very top not to do it," the RMT leader said.
Before the phone call Mr Johnson admitted it had been a "few years" since he had spoken to Mr Crow.
The mayor told him: "Yes, of course there are job losses involved in what we are proposing to do and we accept that. But there are no compulsory redundancies, Bob, and we have already had more than 1,000 people show an expression of interest in voluntary redundancy.
"A huge proportion of those have a firm intent to seek voluntary redundancy. We could go now with those numbers and through natural wastage we would have solved the problem.
"There is absolutely no need to go ahead with this action. So please, please, please get into negotiation with our team."
But Mr Crow said the RMT and TSSA could not negotiate "while you've put a gun to our heads ... you served the notice on our unions to say these jobs are going".
"If you had never served the notice, we would not have been voting for strike action, there wouldn't be a strike tonight," he said.
"The simple way round it is technically withdraw that notice, suspend the notice, we'll suspend the action and we can all get round the table, out of the pressure cooker, and we look at the future of London Underground."
Mr Johnson accused him of "muscle flexing", saying: "The RMT know they have absolutely no chance of stopping this, it's inevitable, but what they need to do is show to their members and indeed to their future possible members that London Underground can't make changes without this kind of industrial action."
Mr Crow and Mr Cortes went into City Hall and said they wanted to arrange a meeting with the mayor. A woman from the mayor's office met them but nothing was arranged.
Mr Crow said: "We have been to City Hall, made ourselves available at any location of his choosing and he has point-blank refused to meet with us. As a result, the strike goes ahead."
Labour's shadow transport secretary Mary Creagh said: "Nobody wants strikes and both sides should now get back around the negotiating table and sort this out as a matter of urgency.
"It is disappointing that the mayor and Transport for London have not reached a resolution with the unions ahead of this strike."
The chief executive of a central London casino attacked the strike, saying it would hit an industry exhibition being attended by representatives from around the world.
Simon Thomas, chief executive of Hippodrome Casino in Leicester Square, said it was "frankly embarrassing" that the exhibition at the ExCeL Centre would be disrupted by the industrial action.
"Gaming industry executives from across the world are in London this week and it is outrageous that this strike is being held.
"Just when the economy is recovering we have this prehistoric attitude, which is not welcome."