More Labour MPs have said they will not contest their seats as Theresa May seeks a larger Conservative majority at a snap general election, amid anxiety within the party about the prospect of losing seats on 8 June.

Labour has confirmed that all sitting Labour MPs will be automatically reselected but a number are considering whether to withdraw from politics just two years after winning their seats in 2015.

Iain Wright, the respected chair of the House of Commons business committee, said he was not standing again because he wanted to pursue other opportunities, while Pat Glass, the former shadow Europe minister, said she wanted to spend time with her grandchildren.

Andy Burnham, who is standing to be Labour mayor of Greater Manchester, said he would stand down in Leigh regardless of his mayoral bid, and Andrew Smith, the longstanding MP for Oxford East, said it was time for someone else to do the job after his 30 years in the Commons.

Gisela Stuart, MP for Birmingham Edgbaston, refused to say if she would stand again, while Fiona Mactaggart, MP for Slough, was also thought to be considering whether to step down.

Alan Johnson, the former Labour home secretary, and Tom Blenkinsop, MP for Middlesbrough South and East Cleveland, were first out of the blocks to say they would not be standing again.

Seats are usually easier to win as an incumbent, so a large number of retiring MPs could increase the number of seats vulnerable to a challenge from other parties.

Gisela Stuart, the Birmingham Edgbaston MP, has declined to say whether she will stand again.

However, it will also give the Labour leadership greater opportunity to support candidates more sympathetic to Jeremy Corbyn. David Prescott, an aide to the Labour leader and son of the former deputy prime minister John Prescott, is considering running in Johnson’s West Hull seat.

Labour’s ruling national executive committee met on Wednesday afternoon to discuss how to approach the election, including candidate selection and compilation of the manifesto. It is understood Corbyn made clear at the meeting he was not considering any progressive alliances with other parties.

Corbyn told his MPs at a parliamentary meeting on Tuesday night that he “does not underestimate” the challenge facing Labour but said the party was ready for the contest and would vote in favour of a poll on 8 June.

But the mood among Labour MPs was gloomy in the House of Commons bars, with some referring to fellow MPs in marginal seats as “our nearly departed members” and working out “what shade of toast” they were in relation to their slim majorities.

Senior party figures described the situation as “catastrophic” and mooted possible scenarios in which there are breakaway factions linked to the Cooperative movement or a rebrand as Old Labour.

The Fabian Society, a group affiliated to Labour, has said its low-case scenario is a loss of 40 seats but that number could be higher and MPs were considering possibilities from 20-100 losses.

However, John McDonnell, the shadow chancellor, said he believed Labour would form the next government after an election that people felt was unnecessary.

He said there would be a number of radical policies to inspire the electorate, including a fairer tax system that meant big corporations and the rich – defined as those earning more than £70,000-£80,000 a year – contribute more.