Corbynite Labour MP Dan Carden has resigned from the shadow cabinet to vote against the so-call "Spy Cops bill".
In a letter to opposition leader Sir Keir Starmer on Thursday, Carden said he intended to go against the Labour whip line to abstain on the Covert Human Intelligence Services (CHIS) Bill "as a matter of conscience."
"I must use my voice and my vote on behalf of my constituents to object to legislation that sets dangerous new precedents on the rule of law and civil liberties in this country," Carden wrote.
The bill, which will face its crucial third reading vote on Thursday, would sanction undercover police officers, MI5 counter-intelligence agents and informants to commit crimes in pursuit of investigations.
The Liverpool Walton MP noted he had followed the Labour whip and abstained at the second reading, hoping to persuade the party to oppose it at the next vote.
"As a Liverpool MP and trade unionist, I share the deep concerns about this legislation from across the labour movement, human rights organisations, and so many who have suffered the abuse of state power, from blacklisted workers to the Hillsborough families and survivors," Carden wrote.
In 2016, the UK's College of Policing issued new guidelines against covert agents forming sexual relationships with members of the groups they infiltrate, following the revelation that undercover officer Mark Kennedy had continued a six-year relationship with one woman.
With the Conservative government holding a Parliamentary majority of 80 since last December's general election, Labour's support or opposition for legislation is insignificant without a major Tory back-bench rebellion.
Carden, first elected to his seat in 2017, is a member of the Socialist Campaign Group of 33 left-wing Labour MPs. That group includes former leader Jeremy Corbyn, former shadow chancellor John McDonnell and former shadow home secretary Diane Abbott, all three of whom voted against the bill at its second reading on October 5.