Matt Hancock said the United Nations had written to him to explain that a technicality in its rules meant it could not offer him a special representative role as planned.
The former health secretary had been due to take up an unpaid post in the UN Economic Commission for Africa to assist countries on the continent to recover from the coronavirus pandemic.
But, after a UN spokesman was quoted saying the job was “not being taken forward”, Mr Hancock released a statement to clarify why the offer had been withdrawn.
Mr Hancock said: “I was honoured to be approached by the UN and appointed as special representative to the Economic Commission for Africa (ECA), to help drive forward an agenda of strengthening markets and bringing investment to Africa.
“The UN have written to me to explain that a technical UN rule has subsequently come to light which states that sitting members of parliament cannot also be UN special representatives.
“Since I am committed to continuing to serve as MP for West Suffolk, this means I cannot take up the position.
“I look forward to supporting the UN ECA in their mission in whatever way I can in my parliamentary role.”
There are previous instances of sitting MPs taking up UN posts, with former prime minister Gordon Brown appointed a special envoy in 2012.
Mr Hancock, who resigned from the Cabinet in June after admitting he broke coronavirus restrictions during relations with an aide, announced on Tuesday he had been offered the position with the global organisation.
But Pass Blue, an independent organisation covering the UN, quoted UN spokesman Stephane Dujarric saying: “Mr Hancock’s appointment by the Economic Commission for Africa is not being taken forward.
“ECA has advised him of the matter.”
Reacting to the announcement that Mr Hancock would no longer be taking up the post, Nick Dearden, director of campaign group Global Justice Now, said: “It is right for the UN to reconsider this appointment.
“If Matt Hancock wants to help African countries recover from the pandemic, he should lobby the Prime Minister to back a patent waiver on Covid-19 vaccines.
“If he’d done that when he was in government, tens of millions more people could already have been vaccinated across the continent.
“The last thing the African continent needs is a failed British politician.
“This isn’t the 19th century.”
The original announcement was criticised as it came on the same day a damning report from MPs was published on how errors and delays by the Government and scientific advisers cost lives during the pandemic.
In a letter posted on Twitter by Mr Hancock on Tuesday, the under secretary-general of the UN, Vera Songwe had originally said that the West Suffolk MP’s “success” in handling the UK’s pandemic response was a testament to the strengths he would bring to the role.
After making the initial announcement on social media, Mr Hancock was congratulated by a host of ministers, including Foreign Secretary Liz Truss and Culture Secretary Nadine Dorries.
Mr Hancock’s time as health secretary was cut short four months ago after leaked CCTV footage showed him kissing an aide, in breach of social distancing rules that he had helped establish.