The RMT Union - which is organising the industrial action - confirmed that the walk outs would go ahead today (August 2).
Now it has been announced that 400 London Overground workers - who are employed by Arriva Rail London - are also set to go on strike over a pay dispute.
This means it is likely that all TfL lines apart from the Elizabeth line, DLR and Trams will be affected.
The strike is in between two national train strikes - also organised by the RMT Union - which are on August 18 and August 20.
These have caused chaos across the country, with only around a fifth of trains running on other national strike days.
So it is likely to be several days of transport misery for Londoners - who have been hit by repeated industrial action over the past three months as the cost of living crisis worsens.
Why is the RMT organising a Tube and London Overground strike on August 19?
Mr Khan and TfL have been negotiating with the Department for Transport about a long-term funding settlement, with the mayor only able to agree short extensions throughout the summer.
The issue has been around lost revenue due to the pandemic, and Mr Khan says if the government doesn’t step in and help Tube and bus services will have to be cut.
The RMT has accused TfL of refusing to share details of the “draft government proposal they received regarding funding of the transport system in the capital, in secret and without any discussion with their recognised trade unions”.
It added the London Underground and TfL management have “consistently refused to engage in discussions around safeguarding jobs, pensions and conditions on the spurious grounds that they are unable to give any assurances to our members until they have a financial settlement with the government”.
The union says that London Overground workers have rejected a “a paltry 5% pay offer” from Arriva Rail London, “despite the company making millions for shareholders”.
RMT general secretary Mick Lynch said: “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.
“TfL have had ample opportunity to be transparent about the funding they will receive and to give tube workers the assurances they need.
“Yet they have totally failed to give those guarantees.
“And Arriva Rail London, a company swimming in money, refuses to give our members a pay rise that will deal with the escalating cost of living crisis.
“There will be significant disruption on August 19, but TfL and Arriva Rail London bear responsibility for this break down in industrial relations.”
TfL has been contacted for comment.