London Underground drivers have voted to go on strike after a ballot found 98.8% of union members were in support of industrial action.
Aslef said its members would take action if changes were forced through without negotiation.
It said Transport for London (TfL) and the government wanted to plug the funding gap caused by the pandemic by "slashing staff pensions and tearing up our agreed working conditions".
TfL has been approached for comment.
The government has paid nearly £4bn in bailouts to keep TfL services running during the coronavirus pandemic after a collapse in passenger numbers led to a drastic reduction in its income.
Finn Brennan, from Aslef, said it was an "extraordinary" percentage of drivers who voted in favour of industrial action.
"Management and government now want to plug that funding gap by slashing staff pensions and tearing up our agreed working conditions. But this ballot result shows that our Tube train drivers aren't prepared to let them get away with it.
"Management should be in no doubt that if they try to force through changes to our agreements, working conditions or pensions, there will be hard-hitting and sustained industrial action across London Underground."
He added: "Our members showed huge courage and determination in keeping Underground services operating throughout the pandemic. They do not deserve to be treated like this and are determined - as this vote shows - to protect their working conditions and pensions."
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