The Liberal Democrat leader, Tim Farron, has accused the government of preparing to use international aid money to “plug black holes” in the defence budget, after reviews of defence and aid spending signalled that more funds would be channelled from the Department for International Development (DfID) to projects in other departments.

“Using aid money set aside to help women and children in the most desperate of situations in the poorest nations of the world to plug defence black holes is something that the chancellor should be truly ashamed of,” said Farron, adding that compassionate conservatism was “well and truly dead”.

Farron made the comments before Wednesday’s spending review, in which George Osborne will set out a five-year projection of government spending, including details of aid spending.

The strategic defence review and a review of the government’s new aid strategy, both published on Monday, signalled that a higher proportion of the aid budget would be channelled through departments other than DfID. Money not spent through DfID is exempt from a requirement – set out in the International Development Act 2002 – to be used on poverty-reducing causes.

The government’s new aid strategy says that although “DfID will remain the UK’s primary channel for aid” more aid will be administered by other government departments “to respond to the changing world”.

The strategic defence review announced that another £300m of the aid budget would be added to the £1bn Conflict, Stability and Security Fund, which is overseen by the Foreign Office, Home Office and Ministry of Defence as well as DfID and will help tackle extremism, illegal migration and serious and organised crime. The details of how much of that fund will be administered by each department will be established in Wednesday’s spending review.

The strategic defence review announced the establishment of a £1.3bn Prosperity Fund “to promote the economic reform and development needed for growth in partner countries”. The fund will be overseen by the national security council, which sits within the Cabinet Office, and will be counted towards the aid budget.

In line with UN targets and Nato, the government has committed itself to spending 0.7% of GDP on international aid, totalling £12bn per year, and 2% of GDP on defence.

Ben Jackson, the chief executive of the Bond network of international NGOs, which includes Oxfam and Christian Aid, said that DfID ranked as among the most transparent aid departments in the world and that other departments would have to work hard to attain the same level.

“The government’s new aid strategy says that DfID will remain the UK’s primary channel for aid but more aid will be administered by other departments,” he said. “We’ll be watching closely to make sure the government lives up to its promise that all departments follow the same high standards as DfID on fighting poverty while remaining transparent, accountable and delivering value for taxpayer money.”

Farron said: “Our work to help the poorest throughout the world was a landmark achievement of the Liberal Democrat-Conservative coalition.

“Liberal Democrats made sure this was enshrined in law just before the parliament ended so seeing George Osborne dismantle this is horrifying.

“Meeting the UN 0.7% aid target is a moral commitment which our country had long promised; it is also a key part of Britain’s ‘soft power’. It helped us hold our head high in the world. This damages us, but above all it hurts those least able to help themselves.”

Farron’s accusation comes as the MoD confirmed it would decommission HMS Ocean, Britain’s biggest warship, when it “reached the end of her life” in 2018. The government spent £65m on upgrading the vessel a year ago and there was no mention of the decommissioning in Monday’s defence and security review.