The union RMT representing Tube and Overground workers announced the strike planned for 19 August, which was called last week, will go ahead.
The announcement comes a day before the latest government funding deadline extension expires.
The union confirmed that more than 10,000 workers will walk out as part of two different disputes, with Tube employees going on strike over pensions and jobs while Overground staff over pay.
“This strike action by our members on London Underground and the Overground is yet another demonstration of how transport workers refuse to accept a raw deal,” said RMT’s general secretary Mick Lynch.
“Transport for London (TfL) has had ample opportunity to be transparent about the funding they will receive and to give tube workers the assurances they need.”
RMT called for a walkout last week but had given TfL until today to reassure workers no job reductions and no detrimental changes to pension would be implemented.
Commenting on the decision, Arriva Rail London, which operates the Overground on behalf of TfL, said it was “hugely disappointed” but still remained committed to resolving the issue.
“We know our people are experiencing cost-of-living pressures and have worked hard to provide a pay offer that recognises this, while ensuring Arriva Rail London has a financially sustainable future and our employees have job security for the long term,” said managing director Paul Hutchings.
Mick Lynch warned the industrial action will lead to massive disruption, as the strike will come between a two-day walkout called by 40,000 railway members of RMT working at Network Rail and 14 other operators.
Mayhem will be maximised as station and operational staff at seven companies – including Avanti West Coast, LNER and c2c – will join the walk out.