Jeremy Corbyn faced a backlash after sacking Owen Smith from his shadow cabinet over Brexit.
Mr Smith was replaced as shadow Northern Ireland secretary after calling for a second European Union referendum.
Mr Smith said he had been sacked for his views on the "damage" Brexit will do to the UK's economy and the Good Friday Agreement.
In an apparent message to Mr Corbyn he added: "Those views are shared by Labour members and supporters and I will continue to speak up for them, and in the interest of our country."
Mr Smith, who unsuccessfully challenged Mr Corbyn for the party leadership in 2016, won public support from pro-EU Labour figures - with former Northern Ireland secretary Lord Hain accusing the Labour leader of a "terrible Stalinist purge".
The move came as Mr Corbyn was also under fire from some of his own MPs over the latest anti-Semitism row to hit the party, relating to his apparent support for an artist who painted a controversial mural in London in 2012.
Brexit is one of Labour's biggest fault lines and Mr Corbyn's decision to move against Mr Smith risks creating a deeper rift.
Lord Hain said Mr Smith had been doing a "terrific job" and "in a shadow cabinet with few big hitters he was definitely one".
Chuka Umunna said Mr Smith was "standing up for our principles" and asked: "What has happened to our party?"
Heidi Alexander said Mr Smith "deserves our respect and unequivocal support" adding that Mr Corbyn knew his views on the EU when he appointed him.
Mr Smith's sacking came amid mounting anger over Mr Corbyn's response to a controversy surrounding an allegedly anti-Semitic piece of street art which was being painted over in 2012.
A spokesman for Mr Corbyn said: "In 2012, Jeremy was responding to concerns about the removal of public art on grounds of freedom of speech.
"However, the mural was offensive, used anti-Semitic imagery, which has no place in our society, and it is right that it was removed."
Mike Gapes said that "apparently (in) Corbyn world free speech is allowed for anti-Semites but not for Labour MPs supporting the views of our members and our 2016 Conference Policy on the EU".
MP Luciana Berger hit out at the "wholly inadequate" explanation and colleague Gavin Shuker said: "It's impossible to confront anti-Semitism in our party if this is the response from the very top."