A former adviser to Labour leader Sir Keir Starmer has warned there will be an “explosion” if the party disciplines any frontbenchers joining picket lines in support of striking rail workers.

Simon Fletcher, who also advised former leaders Jeremy Corbyn and Ed Miliband, said there has been “a lot of simmering resentment and irritation” over the party’s current position.

Sir Keir is considering possible disciplinary action after he reportedly ordered frontbenchers not to join picket lines outside stations, as the country faces the biggest rail strike in a generation.

The Conservatives have sought to use the row to claim Labour is on the side of the striking workers, who have caused chaos for millions of commuters, and the opposition leader will be reluctant to give the Government any more ammunition for that attack.

Mr Fletcher said if people are disciplined for the “simple act of offering solidarity” to those striking, he thinks existing resentment among party ranks will “express itself much more strongly” on a local scale.

Asked what would happen if Sir Keir removed frontbenchers from their jobs, or even took the whip away from them, for joining picket lines, he told LBC’s Tonight with Andrew Marr: “I think there would be an explosion.”

He said there had been “very widespread support” for the RMT union in this particular dispute, pointing to the decision of Labour’s Scottish leader, Anas Sarwar, to visit a picket line at Edinburgh Waverley.

“There would be (a) huge explosion if there was disciplinary action threatened against people,” he added.

It comes after shadow Treasury chief secretary Pat McFadden told colleagues that joining picket lines in support of striking workers would not resolve the dispute on the railways.

Asked what his message to Labour MPs choosing to join the protests would be, he told LBC News: “I would say to them, in the end that’s not how this will be resolved.

“It will be resolved by a deal that gets the railways running again, and that’s where the political focus should be.”

Kate Osborne, a parliamentary aide to shadow Northern Ireland secretary Peter Kyle, joined striking workers in Bromley, south-east London, saying: “I’m a trade unionist, I will always stand on the side of the workers.”

Asked if she would face disciplinary action, Mr McFadden told Sky News: “That’s a matter for the whips and for Keir Starmer.”

Labour whip Navendu Mishra also joined a picket line, saying: “As a proud trade unionist, I stand with all workers on our railway network who are taking industrial action to fight for their jobs and keep passengers safe.”

It was understood Sir Keir was waiting until the end of the industrial action before instructing chief whip Alan Campbell to deal with disciplinary issues.

Labour did not say whether or not the MPs could lose their frontbench roles as a result, with a spokeswoman saying: “Unlike the Government, our focus is firmly on the public.”

The opposition party was cautious not to give credibility to the Government’s claims it is playing a role in the strikes, but is also coming under pressure from unions to stand up for workers’ rights.

Sharon Graham, general secretary of the Unite union, told the PA news agency: “The Labour Party was founded by the trade unions and we expect Labour MPs to defend workers, by words and by actions.”

Mr Sarwar, who sits in the Scottish Parliament, headed to the Edinburgh picket line to show his “solidarity”.

“The workers don’t want strikes. The unions don’t want strikes. The public don’t want strikes,” he said.

Shadow minister Baroness Chapman said it was “perfectly reasonable” for the party’s Scottish leader to take a different view from Sir Keir.

She told BBC Radio 4’s PM programme: “Keir Starmer is a prime minister in waiting leading a government in waiting, and it is important that he behaves in that way.

“So when he says that the right thing for the Government to do is to try to resolve this and get parties round the table, he means it, so he has to walk the walk on that and not just say it.

“I think it’s perfectly reasonable for Anas to take a different view. He is the leader of Scottish Labour. He makes his own decisions.”

Instead of giving his unequivocal support to the workers, Sir Keir has sought to blame the Government for the walk-outs, saying he did not want the strikes to go ahead.

But deputy leader Angela Rayner gave her clear backing to the industrial action, tweeting: “Workers have been left with no choice.

“No one takes strike action lightly. I will always defend their absolute right to do so for fairness at work.”

A number of Labour backbench MPs posted on social media from picket lines.

Former shadow home secretary Diane Abbott tweeted a picture of her with striking workers in Seven Sisters in London, adding “(But don’t tell Keir Starmer)” to the caption.

Ian Lavery, the MP for Wansbeck and a former Labour Party chairman, joined a picket in Morpeth, Northumberland, tweeting: “Solidarity with the @RMTunion today and all days.”

Former shadow cabinet minister Richard Burgon said: “We can’t just keep accepting workers’ wages and conditions being driven down so that the profits of the rich are driven up.”