The decision will bring more travel misery for Brits this summer and follows a long-running dispute over jobs, pensions and conditions.
The union had a turnout of 56.5 per cent, of which 96 per cent of those voted for more strike action.
More than 10,000 RMT members were re-balloted, with the now previous mandate set to expire in June. The most recent strike action on the Underground took place on March 15.
RMT Union general secretary Mick Lynch said: ‘I congratulate every single one of our London Underground members for giving us continued industrial leverage at the negotiating table.
London Underground workers have voted overwhelmingly in favour of extending their mandate to continue taking strike action, the RMT union has announced
The union had a turnout of 56.5 per cent, of which 96 per cent of those voted for more strike action
‘TFL cannot continue to simply wish this dispute away and the government which has drastically cut the funding to London transport budgets, shares a great deal of responsibility for this continuing impasse.
‘London Underground workers want a negotiated settlement and are quite prepared to take more strike action over the next 6 months to make that a reality.’
The Government is pressing ahead with legislation aimed at providing minimum levels of service during strikes.
There was a protest outside Parliament on Monday evening when the Bill was voted through by MPs.
It will be debated in the Lords again before becoming law in the next few weeks.
ASLEF, another union which tube drivers are members of, tweeted today that their ‘Ballots are being dispatched today for members in dispute across the country.
‘Make sure you complete and return your ballot asap.’
Alongside the tweet was an image which read: ‘Keep the turnout high and our mandate strong’.
The Government is pressing ahead with legislation aimed at providing minimum levels of service during strikes (pictured: King’s Cross St Pancras underground station closed on March 15)
The RMT union, which is led by Mick Lynch, this month claimed that Tube stations regularly have to close because of staff shortages caused by spending cuts.
The union said that there have been around 130 Tube station closures since the start of April.
One Tube worker said: ‘Staff are vulnerable when they have to inform the public a station is closed. I have been threatened with violence and verbally abused.
‘There are colleagues who are suffering from poor mental health as a result of their treatment.
‘And some of them are having to take long periods of sick leave which only adds to the already dire staffing situation.’
Mr Lynch said this month: ‘The cuts to Tube staff are creating chronic shortages across the network leading to unprecedented station closures.
‘Frustration among the travelling public has boiled over in some cases to nasty threats of violence and verbal abuse of our members, something RMT will not tolerate.
‘If things do not improve, our members run the real risk of being seriously physically assaulted at work.
‘The responsibility for the staff shortages lies with government budget cuts to Transport for London and the London Mayor who is refusing to stand up to ministers suffocating the capital’s transport finances.’