Jo Stevens has quit over the Article 50 vote
Ms Stevens resigned as shadow Welsh secretary in protest at the decision to force Labour MPs to back the triggering of Article 50 in the Commons.
The Cardiff Central MP said: "Theresa May is now leading our country towards a brutal exit with all the damage that will cause to the people and communities we represent."
She follows Tulip Siddiq and Daniel Zeichner in quitting in anger at the Labour leadership over the last 24 hours.
It follows the party ordering its MPs to vote with the Government on the issue.
Referring to his constituency, Mr Zeichner, who was transport minister, said: "It's my strongly held personal position and I represent three quarters of the people of Cambridge.”
Ms Siddiq said: "I feel that the most effective place for me to counter Theresa May's hard Brexit is from the backbenches."
She added: "I do not support the triggering of Article 50 and cannot reconcile myself to the frontbench position."