Boris Johnson has waged war on trade union leaders ahead of tube strikes that could bring London to a standstill.
The mayor said it was "absolutely outrageous" that London was being "held to ransom" and mocked union baron Bob Crowe for being pictured over the weekend sunning himself in Brazil.
Last-ditch negotiations take place on Monday before the first of two 48-hour walkouts begins on Tuesday.
Transport for London bosses have warned that there will be disruption and told people to get home as soon as possible.
Writing in the Telegraph on Monday morning, Johnson dismissed the action over ticket office closures on the Underground as "politically-motivated muscle-flexing" by the unions in the hope of attracting members.
With fewer than half the RMT members taking part in the ballot and just 30% of the membership voting in favour action, he said: "It is absolutely outrageous that London, the motor of the UK economy - now contributing 25% of GDP - should be held to ransom by this tiny minority."
Photographs were published on Sunday showing Crowe, the general secretary of the Rail, Maritime and Transport Union (RMT) on a Brazilian beach while on a cruise holiday just days before the first strike is due to start.
Boris wrote: "I love a beach holiday myself; like Bob, I have some groovy swimwear that doesn't always meet with acclaim; and I consider it the right of every freeborn Englishman to drink a bottle of wine and turn the colour of a lobster in the sun. Bob Crow is entitled to his holiday," he wrote.
"What he should certainly not be able to do, in my view, is prepare to disrupt the lives of millions of people who are not on holiday but who want to work; indeed, who need to get to work tomorrow to earn the bread for themselves and their families - the bread that will allow them to pay for a holiday of a kind that Bob has been enjoying."
Members of the RMT and the Transport Salaried Staffs Association are due to walk out for 48 hours from 9pm tomorrow and again a week later, in protest at the closures and the loss of hundreds of jobs.
Johnson has offered to hold talks, but only if the industrial action is called off.
The two sides met at the conciliation service Acas last week, with little sign of a breakthrough.
Crow said: "The RMT remains totally committed to making progress through those negotiations.
"In the meantime I have had further correspondence with Boris Johnson and we are available for direct talks with the mayor with no preconditions.
"We are awaiting his confirmation that he is up for getting round the table with no preconditions or caveats."
London Underground has announced contingency plans for the strike days, but passengers face travel chaos if the action goes ahead.
Transport for London warned that Tube services will be hit if the strikes go ahead as planned.
In a statement, it said: "Should the RMT and TSSA strike action go ahead, London Underground will keep as many stations open and run as many trains as possible.
"Services will be affected from 9.30pm on Tuesday February 4 so customers are asked to try to complete journeys as early as possible that evening.
"If your Oyster card needs topping up, it may help to do this in advance of the industrial action starting.
"There will be disruption to journeys on Wednesday and Thursday, when Tube services will start later and finish earlier than normal - trains will run from around 7am to 11pm."
London Underground managing director Mike Brown said: "My message to the union leaderships is that it's not too late to do the right thing, call off this completely unnecessary strike and work with us to help shape the future of the Tube.
All a strike will achieve is to lose those who take part four days' pay.
"Should the strike go ahead, we'll do all we can to keep customers informed and ensure we keep London moving and open for business."