Until now it had been thought that up to 60 would defy the party leader.
But sources say the number of rebels now stands at 80 – more one in three of the 229 Labour MPs in Parliament.
It comes after Mr Corbyn ordered his party to vote for the bill – which will give the Prime Minister the authority to begin the process of leaving the EU – fearing that rebelling against it would infuriate Labour supporters who voted Brexit.
More than a third of Labour MPs could vote against the Brexit bill
Shadow Welsh secretary Jo Stevens resigned on Friday ahead of next week’s first vote in the Commons
Suggesting that there will be 50 or 60 rebels is, pardon the pun, rather conservative. It’s looking like closer to 80
Labour has been plunged into disarray by the rebellion, with shadow Welsh secretary Jo Stevens resigning on Friday ahead of next week’s first vote in the Commons.
She became the second frontbencher in 48 hours to stand down after shadow education minister Tulip Siddiq also decided she could not support the Bill. Daniel Zeichner, shadow transport minister, shadow foreign office minister Catherine West and whips Jeff Smith and Thangam Debbonaire have all said they will vote against Article 50.
Theresa May's 12 point Brexit plan
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Continued cooperation with the EU to tackle terrorism and international crime.
Shadow education minister Tulip Siddiq stood down as she could not support the Bill
An insider said: “They are just the tip of the iceberg.
“Suggesting that there will be 50 or 60 rebels is, pardon the pun, rather conservative. It’s looking like closer to 80.”
When MPs voted on the timetable for Article 50 in December, 89 voted against Brexit and 60 more abstained, including 56 Labour MPs.