Jack McConnell should quit his seat at Holyrood in the first half of next year if he is to take up his new post as High Commissioner to Malawi, according to a powerful Westminster committee.

The former First Minister would force a by-election in Motherwell and Wishaw as soon as he stands down to take up the diplomatic post. Although it is in Labour's Lanarkshire heartlands, the party's current difficulties could make it vulnerable to the SNP.

The Foreign Affairs Select Committee of the House of Commons issued its report into Mr McConnell's appointment yesterday, saying it was "very unsatisfactory" if his move to the African country should be delayed to suit Labour's electoral timetable.

The party had hoped to run the Motherwell and Wishaw ballot on the same day as the Westminster General Election, but that has been delayed and is now expected in spring 2010.

Mr McConnell's appointment as High Commissioner, Britain's most senior diplomatic post in the Commonwealth nation, was announced in August last year, as he stepped down from the Labour leadership at Holyrood. He was due to replace Richard Wildash, the current career diplomat whose term in the Lilongwe High Commission comes to an end next January.

But Mr McConnell told the Foreign Affairs Committee there is no clear plan for him to take over immediately after Mr Wildash leaves. He even suggested there might be someone else appointed temporarily. "Circumstances could change over the next few months that might lead to a different skills set being needed for a certain time," the MSP told MPs.

The Labour-majority Commons committee welcomed the commitments made by the former First Minister to his new role in Malawi, having already forged a renewed relationship with the former colony with strong links to Scotland.

However, yesterday's report concluded: "It would be very unsatisfactory for there to be a lengthy interregnum in which no High Commissioner is in post. It would be equally unsatisfactory for Mr McConnell to continue to perform the duties of a Member of the Scottish Parliament, at the same time that he is preparing to undertake the politically impartial duties of a British diplomat".

It added: "We recommend that the Foreign and Commonwealth Office and Mr McConnell should resolve this issue swiftly, and make a public announcement that Mr McConnell will take up the post on a specified date within the first half of 2009."

A Foreign Office spokeswoman said there will be a response to the report within two months, while Mr McConnell commented: "The handover arrangements for the post are entirely a matter for the Foreign Office and in the meantime, my priorities remain serving the people of Motherwell and Wishaw, and my voluntary work advising the Clinton Hunter Development Institute in Malawi and elsewhere".

While neither the SNP nor Labour has a candidate in place for a by-election, Nationalists used the report to put pressure on Labour. SNP regional MSP Christina McKelvie said: "Jack McConnell and the Labour Party have been caught on the hop by the Foreign Affairs Committee. If Gordon Brown hadn't botched the election last October, Jack McConnell would be long gone. Now he is hanging on to spare Labour's blushes.

"That Labour is running scared of a by-election in one of their strongest constituencies shows just how much Scotland has changed in the last 12 months."

A Labour spokesman responded: "We understand why the SNP are keen for an experienced parliamentarian like Jack to not be in the Scottish Parliament, holding them to account."