Boris Johnson must “slaughter sacred cows” by merging the Foreign Office (FCO) and Department of International Development (DfID), the former international trade secretary Liam Fox is to say in a major speech pushing a proposed Whitehall shakeup.

Ministers have been considering a merger of the FCO and DfID for months, but in recent weeks it was reported that the plans advocated by Dominic Cummings, Johnson’s chief strategist, had been put on hold.

Fox, still influential in Conservative policy making, is to urge Johnson not to grant a last-minute reprieve to DfID in an attempt to buy political peace.

In a speech to the Institute for Government on Tuesday he will argue “the Conservative victory has created a political moment which is as important as Mrs Thatcher’s victory in 1979”, and as a result the reshaping of Whitehall must be as radical.

The foreign secretary, Dominic Raab, has been pressing the case for a single merged department, but the resistance has come from DfID, which is concerned that the key poverty reduction goals of the aid budget will be blurred if its work is conducted inside a politically directed FCO.

Raab confirmed in the Commons that ministers were preparing a root and branch foreign security and development review as part of the post-Brexit landscape.

Fox will claim in his speech that the Foreign Office has seen “decades of degradation of the UK’s overseas capability as the Treasury has taken advantage of Britain’s Eurocentricity to penny pinch”.

He continues: “Unsurprisingly, the selling of global property assets has had a cumulative diminution in Britain’s standing abroad with no benefit, economically or politically at home. When we returned to office in 2010, having worked as a Foreign Office minister in John Major’s government, I was horrified at how hollowed out the FCO had become. This process must be reversed.”

Fox will deny accusations that a reorganisation is a distraction, arguing it is central to ensuring the UK has a single coherent overseas policy. “With a new government elected, with a strong majority, in a Britain about to abandon some of the institutions to which it has been shackled for the last 40 years, this is no time to give undue weight to traditional self-interest.”

Fox will say DfID is “undoubtedly regarded as a hugely successful global brand – just not a British one, despite the valiant efforts of recent secretaries of state”.

He will say: “I was particularly struck at the last Commonwealth heads of government meeting in London where African leaders, in particular, were keen to talk to DfID officials rather than their Foreign Office counterparts.

“A country can have only one foreign policy, not two, and this is an opportunity to correct the problem without undermining our legal commitment to spend 0.7% of GDP on overseas aid.”

Fox complains that the UK currently has more DfID staff in Kenya than his old Department for International Trade has in the whole of Africa.

Fox will claim that those advising against reorganising Whitehall will be “the very same people who sought to thwart Brexit itself”.