The shadow minister joined striking rail workers despite his party leader asking frontbench MPs not to.
Unions have reacted angrily to the decision by Labour leader Sir Keir Starmer to sack a junior shadow transport minister who joined striking rail workers on a picket line.
Ilford South MP Sam Tarry attended the protest at London’s Euston station despite Sir Keir telling his frontbench MPs to stay away.
Labour said the reason he was fired was for unauthorised media appearances.
Mr Tarry said he had been standing “in solidarity with striking workers”.
In a statement, Labour said it would “always stand up for working people fighting for better pay, terms and conditions at work”.
“This isn’t about appearing on a picket line. Members of the front bench sign up to collective responsibility. That includes media appearances being approved and speaking to agreed frontbench positions.
“As a government in waiting, any breach of collective responsibility is taken extremely seriously and for these reasons Sam Tarry has been removed from the front bench.”
But the leaders of a number of unions have criticised the party for the decision.
Sharon Graham, leader of Unite the Union, which has more than a million members, said Labour was “becoming more and more irrelevant to ordinary working people”.
The TSSA Transport Union, of which Mr Tarry is a former member, said Labour was “deluded” if it thought it could win the next election “while pushing away 7 million trade union members”.
Gary Smith, general secretary of the GMB Union, tweeted that it was a “huge own goal” for Labour to “turn a Tory Transport crisis into a Labour story”.
He added that the next election would be fought on the “back of the worst cost-of-living crisis in a generation” and that the “mood of the country is with us”.
Sam Tarry’s sacking risks bringing tensions within the Labour Party to the fore.
The party is pointing to rogue media appearances this morning as the reason – rather than his presence on the picket line itself – but the leadership doesn’t want to take the side of striking workers in this dispute.
Labour wants to be seen as a government-in-waiting.
Both Tory leadership candidates are talking tough on cracking down on strike action – and Sir Keir Starmer knows he needs to win over Tory voters at the next general election.
But Labour distancing itself from striking workers and unions risks alienating its traditional support base and the left of the party – and it needs support from all corners to get into government.
Several Labour MPs, including John McDonnell, Rachel Maskell, Mick Whitley, and Kim Johnson, have also suggested Mr Tarry should not have lost his job.
Ms Maskell said Labour MPs are in Parliament to be the voice of workers seeking job security, better safety, and decent pay.
Speaking while at the protest on Wednesday morning, Mr Tarry told the BBC he was there “as a shadow transport minister backing transport workers who are on strike” and “backing the travelling public”.
Asked if he should be on the picket line. he said “any Labour MP, any Labour member, will have absolute solidarity with striking workers.”
Speaking to BBC Radio 4’s Today programme earlier this week, Sir Keir lsaid that “a government doesn’t go on picket lines, a government tries to resolve disputes”.
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Responding to his dismissal, Mr Tarry – a supporter of the former leader Jeremy Corbyn – thanked Sir Keir for “the last two and a half years” on the front bench, but said it was “a real shame” he had been removed for “joining a picket line”.
He said he wanted to be “part of a Labour Party that stands in solidarity with workers in their disputes, wherever that may be in this country”.
Train services were disrupted throughout Wednesday after 40,000 rail workers walked out in protest at pay, pensions, and working conditions.
Negotiations between the RMT union – which is not affiliated to Labour – the TSSA and Network Rail have failed to find a solution to the dispute.
Labour has not officially supported the industrial action, but has criticised the government, which it says should get involved in negotiations.