Prominent pro-EU MPs seeking re-election such as David Lammy and Margaret Beckett underlined their commitment to fighting to stay in the EU, with more signatures expected ahead of the election on 12 December.
If elected, Labour will seek to negotiate a new Brexit deal with greater protections for workers, before putting that deal to the public in a Final Say referendum.
But Mr Corbyn has refused to confirm whether his top team could campaign for different outcomes until after the negotiations have taken place.
Senior shadow cabinet figures such as Sir Keir Starmer and Diane Abbott made it clear they would campaign for remain in another referendum.
The party’s stance aims to appeal to both sides of the Brexit divide – but it risks leaving Labour vulnerable to the Liberal Democrats who are campaigning to scrap Brexit entirely.
Wes Streeting, who is battling to be re-elected in Ilford South, said: “It’s important to be honest with voters about where I stand before the general election.
“I have always fought for them to have the final say on Brexit and I continue to believe that we’re much better off remaining in the EU.”
Alison McGovern, standing in Wirral South, said she was clear that every kind of Brexit was “negative for Britain”.
“We need a new agenda for remaining and reforming Britain and the EU, and that is why people should vote Labour in this General Election,” she told The Independent.
Some Labour candidates are also under pressure from electoral pacts by the Liberal Democrats, the Greens and Plaid Cymru, who are uniting behind a single candidate in some 60 seats.
Jo Stevens faces a Lib Dem candidate in Cardiff Central after the Greens bowed out – despite being a long-standing campaigner for another public vote.
Proud to have to have signed the @Remain_Labour pledge. Labour is committed to a confirmatory referendum, to give you the final say on Brexit. If re-elected as your Labour MP, I pledge to campaign to Remain in the EU pic.twitter.com/YrW0G7FIPq— Vicky Foxcroft (@vickyfoxcroft)November 8, 2019
She tweeted: “My track record as a leading pro remain MP is clear. Cosy deals between other parties just opens the door to Johnson and his hard Brexit Tories.
“They were second in Cardiff Central in 2017. Shows that this isn’t about making sure we remain at all.”
Thangam Debbonnaire is another Labour candidate facing a challenge from a Remain pact, this time in Bristol West.
She told The Independent: "I know that remaining in the EU is the best option for Bristol West and have consistently promoted this as my preferred option. Therefore I will campaign for remain.
"But it is right for the other option to be a Labour deal based on Labour values, because if the UK votes again to leave, it's vital that it is on terms which protect rights and protections and jobs and trade, rather than undermine them all, as both Tory deals have done and leaving with no deal does even worse.
Therefore I support Labour offering a public vote, negotiating a Labour deal to put on the ballot paper versus remaining in the EU, and asking the people to have the final say."
Andrew Lewin, founder of Remain Labour, which co-ordinated the letter, said Mr Corbyn's party offered the best route to staying in the EU.
He said: “The party is already committed to putting Brexit back to the people and I am delighted that more than 50 parliamentary candidates have already pledged to campaign to remain in the EU in all circumstances.
“Remain voters want to vote for the Labour party with confidence at this election. Our pledge will help candidates in England, Scotland and Wales to underline that the Labour party is the strongest remain alliance in the country.”
At a campaign event in Harlow, Essex, earlier this week, Mr Corbyn said his strategy was to try to “bring people together” over Brexit.
He said: “Our conference in September in Brighton came together and agreed that position.”
The Labour leader added: “At the conclusion of those three month negotiations, we will hold a special conference of our party in a democratic way. We will come to a view as a party.
“At that point we will give that view to the people of this country.”