Shadow transport minister Sam Tarry joined striking workers on the picket line at Euston Station in London, in defiance of Sir Keir Starmer's orders to stay away.
He told ITV's Good Morning Britain: "If we don't make a stand today, people's lives could be lost.
"Some of the lowest-paid workers are on strike today in the rail industry, safety critical workers, workers who make sure our railways get people to work and do so safely.
"It can't be accepted anymore, that people just have to accept that inflation is out of control. The Government's doing nothing on the cost-of-living crisis."
Asked whether he expected to be sacked by Sir Keir, Mr Tarry said: "I've no idea what Keir will decide to do but I know this - if Keir was in government right now, this dispute wouldn't be happening."
'If we don't make a stand today, people's lives could be lost.'
— Good Morning Britain (@GMB) July 27, 2022
Shadow Transport Minister Sam Tarry has joined the picket line as rail workers strike.
He explains why he joined the picket line after Sir Keir Starmer told GMB that Labour MPs should not take part. pic.twitter.com/PWwVWWeNNO
He added: "I have absolutely 100% confidence that any Labour Party MP would be in support of striking workers who have given up a day's pay, a week's pay or even longer."
The Telegraph understands that no explicit order was given by Sir Keir to shadow ministers over Wednesday's strike, but that front benchers were expected to refrain from demonstrating after being banned from pickets during strikes last month.
Anneliese Dodds, the Labour Party chair, said Mr Tarry would be investigated by the party's whips for his apparent defiance.
"Personally I will not be on the picket line because I am a politician and I believe what politicians should be doing now is what the Conservative government has appallingly failed to do," she told Sky News.
"Ultimately it's a matter for that individual, but I am sure that the whips will be looking at this in terms of it being a disciplinary matter."
Live on @GMB - @SamTarry going against party orders and joining the picket line here at Euston. pic.twitter.com/haygeHJDCy
— Brad (@bradleyconnorj) July 27, 2022
A spokesman for Ms Dodds denied reports that she had privately assured Mr Tarry there would be no action against him.
Mr Tarry has previously been romantically linked with Angela Rayner, the party's deputy leader and shadow secretary of state for the future of work.
Ms Rayner, a former union official, has so far supported Sir Keir's stance on the rail strikes but has previously said it is "only because of the Labour Party and our trade union movement that I've gone from no GCSEs and a minimum wage job to where I am today".
On Wednesday she tweeted: "Labour will always defend the right of working people to organise and withdraw their labour - to stand up for services, public safety, defend jobs and pay."
The pair were spotted leaving Ms Rayner's London flat together in January, but her spokesman refused to comment on whether they were still in a relationship.
In response to the move, Grant Shapps, the transport minister, told Sky News: “It’s clearly in defiance of Sir Keir Starmer, who’s told his front bench they shouldn’t be. He’ll no doubt want to remove him from his job.
“Nobody should be on the picket lines, stopping hardworking people who spent £160,000 per railworker preventing any of them from losing their jobs during the pandemic. We come out of the pandemic and this is the way people are being thanked.
“If Labour frontbenchers want to go and join the on the picket line, people will come to their own conclusions and I’ve no doubt Sir Keir Starmer will want to sack him.”
Mr Tarry's actions come in defiance of Sir Keir, who last night said he will again tell his shadow frontbench not to go on the picket lines outside railway stations during upcoming strikes.
The Labour leader told ITV's Good Morning Britain: "It's quite open to people to express their support for working people who are struggling to pay their bills, but I'm very clear that the Labour Party in opposition needs to be the Labour Party in power.
"And a Government doesn't go on picket lines, a Government tries to resolve disputes...
"I'm so frustrated with our Government because they could step in and help solve the dispute... I think the Government just wants to feed on the division".
'The Labour party is in a pickle'
On Wednesday morning Mick Lynch, the general secretary of the RMT union, accused Sir Keir of abandoning working class voters and trade unionists by refusing to support the strike.
“The Labour Party has got itself in a pickle, haven’t they?” he told BBC Radio 4’s Today programme.
“They don’t seem to know which direction they’re facing and Keir Starmer is backing away from his commitment on public ownership and all sorts of stuff.”
“Most of the MPs in the Labour Party want to support us instinctively and naturally because they are from the trade union movement, every one of them.”
“Keir Starmer needs to get in tune with where working class people is [sic], because they are being ripped off in society at the minute.”
“Wages are being suppressed, conditions are being ripped out, and people feel there is no balance in the workplace.”
Mr Lynch added that Sir Keir should “work out a set of principles and policies that appeal to working class people in all those communities that the Labour Party lost at the last election”.
