Labour MP Alan Johnson Reuters / Luke MacGregor LONDON — A total of 12 Labour MPs have stood down ahead of the summer general election.

Labour MPs had until 6 p.m. GMT Thursday to decide whether they would fight to retain their seats in the June election — and all but twelve said they would be running.

The most high-profile Labour MPs to announce their departure from Westminster politics are former home secretary Alan Johnson and highly-respected former shadow cabinet minister Michael Dugher.

The others are former leadership candidate Andy Burnham who is standing in the Manchester mayoral election, as well as Gisela Stuart, Jim Dowd, Tom Blenkinsop, Rob Marris, David Anderson, Pat Glass, Iain Wright, Andrew Smith, and Fiona MacTaggart.

Liverpool mayoral hopeful Steve Rotheram was expected to announce his resignation but this is yet to be confirmed.

These departures come as Labour leader Jeremy Corbyn gets his election campaign underway. Corbyn said the election was a chance to take on the "establishment" at a campaign launch in central London on Thursday morning.

Corbyn, whose party is predicted to be roundly defeated by Prime Minister Theresa May's Conservatives in June, also ruled out a nationwide referendum on the terms of Brexit's exit from the European Union if he was prime minister.

This was after shadow chancellor John McDonnell suggested May's government should "put the deal to parliament and possibly to the country overall" once agreed with the EU.

"A second referendum is not our policy and it won't be in our manifesto," a spokesman for the Labour leader said.

Meanwhile, former cabinet minister Yvette Cooper is tipped as an early favourite to succeed Corbyn should Labour suffer defeat at the election. Allies of Cooper say preparations for a leadership campaign are already underway.

Chuka Umunna is also considering a leadership bid, according to reports.

Labour was not the only party to lose MPs ahead of the election. The biggest name to announce their resignation was former Chancellor George Osborne, who up until yesterday was both Evening Standard editor and a Tory MP.

Prominent Brexiteer Douglas Carswell has also quit parliament after serving both for the Conservatives and UKIP.

The other MPs to have stood down are Conservatives Gerard Howarth, Simon Burns, and Angela Watkinson plus Lib Dem John Pugh.