Following the Holyrood election defeat Sarwar promised to stay on as leader in Scotland but his appointment as a minister at Westminster is expected to create a leadership vacancy.
Deputy leader of Scottish Labour and MSP Jackie Baillie is expected to step up to cover Sarwar’s role ahead of a contest.
There will not however be a by-election for Sarwar’s Holyrood seat as he was elected via the regional list.
Monica Lennon, who had been in the Scottish Parliament for a decade but was not re-elected in May, is the next name on the Glasgow list.
As of last year, new rules prevent politicians from sitting at both Westminster and Holyrood at the same time.
In May, as reported by The Herald, external, when asked if he would ever take up a seat in the House of Lords, Sarwar said: “No. I’m focused on being first minister of this great country.”
He added: “My only commitment, my only focus, is serving my constituents in Glasgow and serving the great people of this country, if I get the honour of being first minister.”
After Sir Keir’s resignation in June, when asked if he planned to serve as party leader for the full five-year Holyrood term he said: “That is my intention.
“I’m hungry for the fight ahead and I want to play my full part in making sure this country never ends up in the hands of Nigel Farage and Reform.”
