Getty ImagesThe UK's largest teaching union will "make Labour MPs pay a high political price" if the government does not offer teachers in England a fully funded, above-inflation pay rise, its general secretary has said.
Daniel Kebede said members of the National Education Union (NEU) "expect better from a Labour government" than the pay recommendation ministers have made.
He said NEU members would campaign in Labour constituencies if it did not change - and "stand ready to strike".
Education Secretary Bridget Phillipson has said "any move towards industrial action by teaching unions would be indefensible".
The government has recommended a 2.8% pay rise for teachers in England and expects most schools will need to make "efficiencies" to fund it.
The NEU agreed at its conference in Harrogate this week that it would hold a formal ballot on strike action if the offer remains "unacceptable", or if no extra funding is announced.
Phillipson has said: "With school staff, parents and young people working so hard to turn the tide on school attendance, any move towards industrial action by teaching unions would be indefensible.
"Following a 5.5% pay award in a hugely challenging fiscal context, I would urge NEU to put children first."
Responding to her comments, Mr Kebede told conference it was "indefensible for a Labour government – a Labour government – to cut school funding".
"After 14 years of Conservative austerity, we expect better from a Labour government," he said.
He said the final pay award must be above inflation and fully funded, representing a "correction" of teacher pay and helping to recruit and retain staff, otherwise "we stand ready to act industrially".
"We will make Labour MPs pay a high political price through our campaigning in their constituencies, with our parents, across this country," he said.
"No teacher wants to strike, but we stand ready."
Mr Kebede accused the government of a "betrayal" when it came to education funding.
Mr Kebede also commented on a row with Reform UK leader Nigel Farage on Tuesday after the NEU branded Reform UK a "racist and far-right" party.
During the clash, Mr Kebede called Farage a "right-wing populist" but did not describe him as racist, while Mr Farage vowed to "go to war" with the teaching unions if it won the next general election.
On Thursday, Mr Kebede told NEU members that Farage "knows nothing about education".
"And whilst this government might be rolling out the red carpet for Nigel Farage to walk into No 10, through their austerity agenda, we won't stand for it," he said.
