Labour said an "extremely large" number of people have put themselves forward to succeed the late David Cairns as the new MP for Inverclyde.
Twenty-five people have submitted their names for selection in the Inverclyde by-election.
Mr Cairns, who had pancreatitis, died on May 9. He held the seat for Labour in 2010 with a majority of 14,416.
However, in the nearest equivalent Holyrood constituency, Greenock and Inverclyde, Labour's lead slumped to just 511 in the Scottish Parliament elections last month.
Among those declaring their candidacy are recently unseated Labour MSP David Whitton, who watched a notional majority of 3,583 in Strathkelvin and Bearsden melt away last month, handing the seat to the SNP in a 7.69% swing.
Also in the frame is Iain McKenzie, leader of Labour-controlled Inverclyde Council; Frankie Caldwell, a former army captain; and Melanie Ward, a former president of the National Union of Students.
The Labour shortlist will be announced tomorrow and the final candidate will be selected at an Inverclyde Labour party hustings on Saturday.
A Labour spokesman said: "An extremely large number of people, 25 to be exact, have put their names forward on this occasion.
"No decisions have been made on the date of the by-election yet but there has been speculation that it will be held on June 30. This is unlikely as we would not usually hold a by-election in the middle of the school holidays, so it is likely to be July or August.
"We have traditionally allowed quite a long lead-in time for by-elections in the past."