Owen Smith has been sacked from the shadow cabinet after breaking ranks with Jeremy Corbyn by calling for a new EU referendum.
He was replaced as shadow Northern Ireland secretary after saying Labour should keep asking whether Brexit remains the right choice for the UK.
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, insisted Labour needed to do more than "just back a soft Brexit".
Mr Corbyn has been careful to say that Labour is not seeking a referendum on a Brexit deal, but has avoided completely ruling out such a vote.
In a sign of the leader's anger at Mr Smith's intervention, he was asked to stand down from the frontbench and replaced with immediate effect by former minister Tony Lloyd.
Writing in The Guardian, Mr Smith said: "Given that it is increasingly obvious that the promises the Brexiters made to the voters - especially, but not only, their pledge of an additional £350 million a week for the NHS - are never going to be honoured, we have the right to keep asking if Brexit remains the right choice for the country.
"And to ask, too, that the country has a vote on whether to accept the terms, and true costs of that choice, once they are clear.
"That is how Labour can properly serve our democracy and the interests of our people."
Mr Smith said that remaining in the EU single market and customs union was the only way to meet the UK's commitments under the Good Friday Agreement.
"If we insist on leaving the EU then there is realistically only one way to honour our obligations under the Good Friday Agreement and that is to remain members of both the customs union and the single market.
"I'm pleased my party has taken a big step in this direction by backing continued customs union membership, but we need to go further."
Prominent pro-EU Labour MP Chuka Umunna described Mr Smith's comments as "excellent".
Mr Corbyn moved swiftly to replace Mr Smith with Labour veteran Mr Lloyd.
He said: "Tony is a highly experienced former Government minister who is committed to ensuring that peace in Northern Ireland is maintained and helping to steer the devolution deal back on track."
Mr Lloyd said: "As we leave the European Union, ensuring there is no hard border between Northern Ireland and the Republic is of paramount importance and this will be my number one priority.
"This is an incredibly important job, with a huge in-tray and I am looking forward to starting work."