JOHN MCDONNELL has appealed to Conservative MPs to join the fight against “selling the future of our children.”

Speaking at a packed rally of teachers yesterday, the shadow chancellor said budget cuts had pushed schools to breaking point.

Members of six unions descended on Westminster to lobby their MPs ahead of next month’s autumn budget.

Mr McDonnell said: “I hope in the run-up to this budget that the Tories will listen. We will support increases in the education budgets that they bring forward, but it has to be enough.

“This is selling the future of our children, and whatever political party you’re in, you can’t stand by and let this go on.”

Liberal Democrat leader Sir Vince Cable received a more ambivalent reception at the the rally, with one audience member heckling him over his role in raising tuition fees.

But he told the crowd his party would oppose any attempt to fund schools out of existing budgets, saying there must be “a commitment to decent pay” for the workforce too.

“Parents will be divided against teachers,” he said. “This is a potentially wicked outcome of the pressure we are beginning to see.”

Teachers’ union NEU leader Mary Bousted blasted the “terrible conditions” of British school buildings that had resulted from a lack of repairs.

“While the government fiddles [school funding figures], school buildings crumble,” she said.

Parent campaigner Jo Yurky, who co-founded the Fair Funding for All Schools group, said anti-cuts campaigners were “already winning the argument.”

She said: “School funding was a toxic issue for the Conservatives in the election. We forced the government to concede we were right.

“We are winning and we won’t stop until the money is made available for all our schools. It’s what every child deserves and every parent demands.”

Yesterday, Tory donor and academy sponsor Lord Harris became an unlikely ally for school funding campaigners.

He told the Guardian: “Education is the most important thing in the country. It’s the future for children. What I think they should do is somehow put more money back into education.”

He also suggested free schools were a drain on government education resources.