The government will be in clear breach of the law and exposed to a judicial review if it presses ahead with a multibillion-pound cut in the UK’s foreign aid programme, according to legal advice given to Tory backbenchers. Advice issued by the QC and peer Ken Macdonald said No 10 had acted outside the law when it abandoned its commitment to spend 0.7% of national income on aid. His advice was commissioned by senior Tory MPs campaigning to reverse the aid cut. They claim they have a Commons majority if necessary to defeat the government and are demanding a vote before the Easter recess, which begins on 25 March. Andrew Mitchell, a former international development secretary and the leader of the Tory backbench rebellion said: “The legal opinion finding the government’s decision unlawful and that the ‘government’s illegality is clear’ is disappointing but not unexpected. I have been urging ministers to address this in parliament and I believe it is not too late to change course before the start of the new financial year in April.” Tobias Ellwood, the defence select committee chairman, said: “The recruiting sergeants of Hezbollah, al-Shabaab, Boko Haram, Isis and other armed militia will be the immediate beneficiaries of the cuts to the UK’s humanitarian programmes. China and Russia will not hesitate to fill the vacuum we create.” The defence secretary, Ben Wallace, insisted on Sunday that the government has the right by law to cut the budget and would not lose any Commons vote. Ministers have until recently been sending out mixed signals about their legal right to cut the aid budget temporarily from 0.7% of the gross national income to 0.5%. Doing so will reduce it by as much as £2.5bn this financial year, on top of cuts last year caused by the decline in the size of the economy. Ministers seemed to have settled an internal legal debate on their powers last week when they said there was no requirement to stage a Commons vote on the cuts. Boris Johnson has promised that the 0.7% target, set out in the 2015 International Development Act, will be restored as soon as economic circumstances allow, but he has given no timetable or framework. The legal advice from Macdonald, a former director of public prosecutions, says the act does not give ministers latitude to miss the target deliberately, and that it “admits of no exception and leaves no ambiguity. Properly understood the secretary of state’s decision is contrary to the clear expressed intention of parliament”. It says the provisions in the act that allow ministers to report to parliament when the target has not been met only applies when the target has been inadvertently missed, not as a function of deliberate policy. It also says that passages in the act that make ministers solely accountable to parliament and not the courts does not preclude a judicial review. “Ouster clauses” that prevent judicial review have rarely been accepted by the courts, the opinion states. The government will face further embarrassment when it is expected to announce a large cut in its humanitarian aid to Syria when the UN holds its annual donor conference in Brussels at the end of the month. It has also quietly announced, as part of the integrated foreign and defence review last week, that £492m is to be cut from the £1.4bn conflict security and stability fund in this financial year. Much of the fund is dependent on aid spending. Liz Sugg, who resigned from the government over the cuts, said: “Legal opinion like this risks undermining our credibility on the world stage at the very moment we need to strike trade deals, negotiate communiques and agree ambitious legally binding climate change targets. Cutting our aid budget sends a message that we are withdrawing from the world.”