John McDonnell branded the sacking of Sam Tarry, who joined rail unions protesting outside Euston Station, a “severe mistake”.
Shadow Transport Minister Mr Tarry was fired after he gave interviews supporting RMT workers who walked out on Wednesday in a long running dispute over pay, pensions and redundancies.
Mr McDonnell told Sky News: “I don’t know who is advising Keir Starmer, but this is a completely unnecessary row that’s been invented just at a time when the Tories are tearing themselves apart, and we’ve got the maximum opportunity I think to gain an advantage in the polls that will build the support to take us into a government.
“We are having this completely unnecessary row.
“Sam went on the picket lines like minister after minister, shadow minister after shadow minister over the years in support of workers who are asking for a decent pay rise. It’s a just cause.”
Sir Keir Starmer had told his MPs not to appear on picket lines ahead of the industrial action.
But several defied his orders.
MP for York Central Rachael Maskell called for the Labour leader to visit picket lines, while GMB general secretary Gary Smith said it was a “huge own goal” for Labour to “turn a Tory Transport crisis into a Labour story”.
Mr Tarry, who is believed to be in a relationship with Labour’s deputy leader Angela Rayner, was the only MP to be disciplined.
Labour has insisted he was not sacked for joining striking workers, but for making up policy when she said it was “not acceptable” to offer below inflation wage rises because it would mean a real-terms pay cut for workers.
Mr Tarry was told it is the party’s position that it is for ministers and unions to negotiate terms and conditions.
“This isn’t about appearing on a picket line,” a party spokesman added. “Members of the frontbench 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 frontbench.”
But Mr McDonnell rejected the claim.
He said: “Now we are being told he was sacked not because he went on the picket lines but because he made statements on the picket lines.
“But what was he supposed to do? Go on there and wear a gag? It is a silly silly situation to get in to.”
He added that Shadow Chancellor Rachel Reeves was not sacked for “making up” policy last week when she said nationalising transport and energy companies would not meet the party’s “fiscal rules”.
Mr McDonnell said “[Mr Tarry] expressed a view about the dispute, which I think is the correct view.
“Rachel Reeves went on an interview last week and made up policy on rail nationalisation, which had to be contradicted by the shadow spokesperson on transport within hours.
“I didn’t see Rachel Reeves being sacked.
“I just think this is an unnecessary division when we should be united.”
Labour sources have alleged Mr Tarry was motivated to promote himself with unions in his local constituency Ilford South, where he is facing a deselection battle.
However this was branded “absolute nonsense” by people close to Mr Tarry.
One source told the Standard that several shadow ministers had written resignation letters ahead of yesterday’s strikes ready to be published if they faced being sacked for supporting the unions.
With multiple other walk outs planned in the coming weeks and months some senior Labour MPs are said to have expressed alarm that they may be disciplined for expressing solidarity with workers taking industrial action.
“We are a party born out of a the trade union movement,” a Labour source said. “It is ridiculous to see shadow ministers sacked for supporting workers.”