CHANCELLOR George Osborne was left facing one of the toughest tests of his career last night after his own side lined up to condemn cuts to tax credits — and it was revealed that his policies would steal money from impoverished children.

The latest challenge to Mr Osborne came from Tory backbencher Stephen McPartland, who published research proving that low wage households will see their kids’ cash slashed on top of working tax credits cuts worth £1,300 a year.

Mr McPartland followed up the announcement by boycotting a government event in his Stevenage constituency — after a top Tory minister refused to even talk about the cuts.

“Treasury Minister David Gauke is coming to Stevenage to talk about research and development into tax credits,” he said yesterday.

“I am boycotting the meeting and the media are not invited, as he does not want to talk about the cuts to child tax credits I have uncovered.”

Labour shadow Treasury secretary Seema Malhotra said: “It is simply astounding that David Cameron and George Osborne’s failure to address concerns around their proposed tax credits cuts means one of their own MPs has had to protest in this way.”

The Chancellor insists the crucial benefit keeping kids out of poverty will not be affected by cuts to working tax credits.

But House of Commons Library figures blow a hole in his claim by proving that cutting one benefit will affect the other.

A lone parent with two children working 35 hours a week and earning £20,000 would currently be eligible for £5,317 in working and child tax credits.

After Mr Osborne’s changes come into effect in April however the same family would lose their working tax credit — and 36 per cent of child tax credit.

The “massive cut” would leave that family with just £3,133, according to Mr McPartland.

He stormed: “This is completely unacceptable and destroys the government’s final defence that planned cuts do not apply to child tax credits.”

The Chancellor’s latest embarrassment came just hours after the publication of a damning report by the Tory-dominated work and pensions committee, accusing Mr Osborne of going “too far and too fast” with cutbacks and noting that their own research was “hampered by the inaccessibility of data about the effects of both the original proposals and potential mitigations.”

Mr Osborne faced further trouble when Tory MP Heidi Allen demanded he implement Labour’s plan to prevent tax credits cuts.

Calling for the Chancellor to scrap his inheritance tax giveaway to Britain’s richest instead, she told him: “We talk so often about ‘all being in this together’ — now is the time to put that mantra into action.”

Former PM Gordon Brown called for the cut to be scrapped completely as he stepped into the row to make a rare speech in defence of the policy he implemented in 2003 as New Labour Chancellor.

He said: “No phasing in, no temporary halt or slower pacing of the implementation of the Osborne tax credits proposals will make a fundamental attack on the working poor and on children much fairer.

“They are a mistake — root and branch.”