Boris Johnson has been criticised for “siphoning off” from the UK’s foreign aid budget to fund diplomacy in former Soviet states and the Middle East via a new £700m “pro-democracy” fund.

The row came as the foreign secretary embarked on a two-day visit to the Gambia and Ghana, where he will announce that the Gambia is to rejoin the Commonwealth.

The fund is expected to be used to foster stability and shore up western influence in countries threatened by Russia, such as Ukraine. However, questions have been raised after it was revealed the fund could be spent on “empowerment” projects in the Baltics, countries that are not on the list of those eligible for official development assistance, which are only the poorest and lowest middle income countries.

The cross-government £700m fund would be available over four years, a Foreign Office spokesperson said. The FCO said in a statement: “The empowerment fund is in the early stages of development. Details of the fund will be announced in parliament in due course.”

Foreign Office sources said the new empowerment fund would be aimed at promoting democracy in countries across the world.

The Labour MP Stephen Doughty, who sits on the international development select committee, said he was concerned that aid funds were being diverted. “I absolutely support us bolstering the Baltic states and Ukraine against Russia … but there is a concern here if it involves diverting funds from destabilised countries in Africa and the Middle East and elsewhere, where we’ve also got challenges of poverty and other risks to UK national security,” he told BBC Radio 4’s World at One.

“My understanding is that [the Baltic States] are not eligible. Only Ukraine is currently because it covers only poorest and lowest middle income countries. I’m not opposed to the government supporting the Baltic states or Ukraine, I think that’s the right thing to do. But it’s about what budget is this coming from?

“It seems like this is what the Foreign Office or Ministry of Defence should be doing rather than diverting it away from the poorest countries.”

The SNP’s international development spokesperson, Patrick Grady MP, said the UK government was “stretching the definition of aid” to include trade and diplomacy efforts. “Siphoning off aid funds to bolster the Foreign Office or MoD budget is a betrayal of our promises to help people living in poverty around the world,” he said.

“The UK government has rightly been applauded for meeting that 0.7% target of national income for aid spending. But it must not undermine that achievement by stretching the definition of aid and putting its own trade and diplomatic interests ahead of helping the poorest and most vulnerable communities around the world.”

The Conservative MP Nadhim Zahawi praised the scheme as the right target for the UK’s aid budget. “If this sort of fund will help push back and allow communities to be stabilised, then I think ... this sort of work is vital,” he told the BBC. “At the end of the day we have to make sure the aid budget is balanced and I think this is a pretty good way of using it, if that is what it is.

“I saw it first hand If you don’t stabilise those parts of the Ukraine, you end up with people not having an education, not having a livelihood and a pretty miserable situation, which could lead to further escalation of that war.”

During his visit to the Gambia on Monday, Johnson will give a speech about the strength of the Commonwealth, with the Gambia rejoining after the election of Adama Barrow.

Former president Yahya Jammeh had said his country would “never be a member of any neocolonial institution and will never be a party to any institution that represents an extension of colonialism”. Jammeh, who had been criticised by the FCO for his human rights record, was defeated in elections last year by Barrow.

Johnson is to visit new medical research facilities in the Gambia and meet employees from the tourism industry, as well as hold discussions in both countries on security cooperation.