David Cameron condemns 'unjustified' Tube strike
The Prime Minister has attacked planned strikes by London Underground workers.
RMT Union members will walk out for 48 hours on Monday evening and then again a week later in protest at ticket office closures and 960 job cuts.
David Cameron tweeted: "Next week's Tube strike is unjustified and unacceptable. It will hit millions of families across the capital and cause chaos for businesses."
Both sides have said they will continue to meet.
Mick Cash, the RMT's acting general secretary, responded: "Instead of attacking Tube workers who are fighting to defend jobs and safety, David Cameron should be hauling in his Tory colleague Boris Johnson to explain why he has turned his promise to Londoners not to close ticket offices on its head."
London Underground (LU) plans to run as many services as possible during the strikes, but warned there will be disruption from Monday evening, with trains starting later and finishing earlier than normal.
Phil Hufton, chief operating officer at LU, urged the RMT to call off the "unnecessary" strike, adding: "Should it go ahead, we will be working harder than ever to keep London moving and open for business."
Extra bus and river services will be provided, but people were urged to check before they travel.
The strike is likely to hit university students taking degree exams. On its website King's College London says it is aware of students' concerns and is monitoring the situation.
Football fans travelling to Arsenal's game with Newcastle on Monday and Chelsea's Champions League match with Atletico Madrid on Wednesday will also be affected.
The union and LU have been holding talks at conciliatory service Acas.
LU said it had made concessions but the union said the transport authority had refused to budge from their plans.
BBC London transport correspondent Tom Edwards said union insiders had told him the chance of a Tube strike being called off were "zero".
Widespread disruption was caused in February when Tube workers took strike action for two days.