Buses, Tubes and mainline trains were hit by the heavy snow last week
Underground bosses have been accused of "hypocrisy" for telling staff they will not be paid for failing to turn up for work during last week's heavy snow.
London's entire bus network and much of the Tube was withdrawn from service due to Monday's extreme weather.
Union leaders said it was therefore unfair to penalise staff for not coming into work.
Transport for London (TfL) said workers needed to provide a good reason for not attending work last Monday.
Union leaders are holding talks with managers later amid growing anger against bosses and London Mayor Boris Johnson.
The Transport Salaried Staffs Association (TSSA) said its members working in ticket offices and in other grades on the London Underground network have been told they must claim last Monday as unpaid leave or a day out of their annual holidays.
"We think this is gross hypocrisy on the mayor's part," said Manuel Cortes, assistant general secretary of the TSSA.
"He cancels all the buses and most of the Tube and urges Londoners not to risk trying to get to work in the worst snowstorm for a generation."
The RMT's general secretary Bob Crow said: "Anyone who followed the news that TfL suspended bus services because of the snow last Monday will be baffled by its decision to stop the pay of those who couldn't get in.
"People were being advised not to travel - and the fact that a large number of Tube workers got in to work speaks volumes for their commitment - but that is no reason to discriminate against those who could not."
A TfL spokesman said managers did not make a "conscious decision" to cancel any services.
"We recognise that exceptional weather conditions made it impossible for some staff to get to work, but we maintain a strict policy that anyone who can get into work should do so.
"If staff are not able to provide a good reason for why they were not at work, they will be expected to take annual or unpaid leave for that day."
TfL also said that only 100 staff did not make it to work last Monday, about 5% of the total workforce and managers would be considering what to do on a case-by-case basis.
Employment lawyer Richard Nicolle said the move could possibly risk industrial action.
"It will certainly prove unpopular to require employees to take it as enforced holiday but in the employer's view it's less risky than deducting pay from a worker's next salary payment," said the partner employment law firm Denton Wilde Sapte.
"It's therefore more difficult for an employee to take action legally if they are simply being told the day will be taken out of their annual leave.
"However, with a unionised workforce such as the Tube for example there could be the risk of industrial action."