Anti-gay laws in Commonwealth countries are “totally out of date, highly inappropriate and should be changed”, the UK’s deputy foreign secretary has said.

Alan Duncan promised MPs the Foreign Office was “fighting in every capital in the world” to block a surprise African nations-led move at the UN general assembly to end or delay the appointment of a UN expert to champion LGBT rights. Many of the countries trying to block the proposed mandate are Commonwealth members.

The move at the UN was expected to be made either on Tuesday or later in the week, and there had been frustration in the Labour party at the reluctance of the FCO to set out its view in public.

Duncan admitted the FCO had been surprised by the sudden move by the African nations, and only became aware of the plan on Friday, but was resisting it.

Duncan, one of the first openly gay Conservative MPs, promised there was “a complete starburst of UK diplomatic effort” in the FCO to assemble a coalition to defeat the African nations-led move.

Duncan said: “All people are born with equal rights and should enjoy the protection of the UN.” He said “the UK will resist any and all attempts” to block the mandate of the new LGBT champion.

The UN human rights council voted in June to appoint an independent LGBT expert for the first time to monitor “violence and discrimination based on sexual orientation and gender identity” around the world. Prof Vitit Muntarbhorn was handed the role last month and was due to investigate abuse against the global LGBT community.

African nations and some Islamic states said gay rights should not be regarded as a human right, and want his mandate revoked at a meeting of the UN general assembly in a vote scheduled either for Tuesday or Thursday. African nations are also concerned that sexual orientation and gender identity was being given attention “to the detriment of issues of paramount importance, such as the right to development and the racism agenda”, he said.

The push at the UN is being led by Botswana and has the potential to turn into a wider battle over anti-gay laws in the Commonwealth. Although Duncan said the days were over when the UK could instruct Commonwealth nations what laws to adopt, he urged the Commonwealth to stop lagging behind on gay rights. He resisted suggestions that UK aid should be conditional on countries’ stance on gay rights since that may punish innocent people.

The Labour shadow minister for equality, Sarah Champion, wrote to the foreign secretary, Boris Johnson, earlier this week. “I am extremely concerned that African nations are now seeking to initially suspend the UN’s first independent expert charged with investigating violence and discrimination based on sexual orientation and gender identity,” she wrote.

“You will of course understand the strong imperative for such a role to exist and the very real implications for LGBT individuals all over the world who continue to be subject to the most appalling and systemic discrimination, hatred and violence.

“According to a UN human rights report last year, at least 76 countries retain laws used to criminalise and target people on the basis of their sexual orientation and gender identity, including laws criminalising consensual same-sex relationships among adults.”