David Marshall stepped down on grounds of ill health
The resignation of Glasgow East MP David Marshall, who has been in the Commons since 1979, is set to trigger a by-election in a government safe seat.
This has opened up a vacancy in the constituency where Labour holds a 13,507 majority.
Labour strategists were understood to have pencilled in July 24 as a possible date for the by-election.
A party spokesman said it was still deciding when the writ would be moved which officially sparks the contest.
He said: "The National Executive Committee will consider the timetable in due course but at this time our thoughts are with David Marshall and his family.
"David has been an outstanding champion for the people of Glasgow all his life and he will be sorely missed."
Mr Marshall, 67, announced his decision to step down after almost 30 years in the House of Commons on grounds of ill health.
It could spell a further headache for Prime Minister Gordon Brown following the resignation of the Scottish Labour leader Wendy Alexander and recent government by-election defeats in Crewe and Henley.
The treasury announced Mr Marshall's appointment to the nominal post of steward and bailiff of the manor of Northstead.