Labour frontbenchers who defied Keir Starmer to appear on picket lines have been encouraged to issue public apologies or risk disciplinary action, despite some shadow cabinet members urging the Labour leader to drop the issue. Several frontbenchers were pictured alongside striking RMT workers on Tuesday, despite explicit instructions from Starmer to stay away. The Guardian understands some of the rebel MPs involved have been asked to issue a statement explaining their attendance. One senior party source jokingly suggested a junior frontbencher had been asked to claim they had been “ambushed by a picket line” – a reference to an excuse used to explain Boris Johnson’s attendance at his own birthday party. Another source, an MP, said the whips were “threatening people at the moment, trying to get them to issue apologies”. It is unclear what the consequences would be if they refused to apologise. Meanwhile, Starmer is under pressure from some members of his shadow cabinet to drop the threat of disciplinary action. One shadow minister said it would be “outrageous” to caution, or even sack, Labour MPs for showing solidarity with striking railworkers. Another shadow cabinet member suggested it had been the right decision to encourage MPs to stay away from the frontline of the strikes, as Labour tries to look like a future government – but wrong to turn it into a show of strength. No final decision is expected to be made about how to handle the errant MPs until after this week’s RMT strikes are over. Five frontbenchers defied Starmer’s order, which was issued in a memo from the leader’s office on Monday, telling his shadow cabinet they should discuss with their teams the need to show “leadership” by staying away. “Please be reminded that frontbenchers including PPSs [parliamentary private secretaries] should not be on picket lines,” the message said. In the event, three PPSs, one junior minister, Alex Sobel, and a whip, Nav Mishra, posted pictures of themselves attending the first of three days of industrial action on Tuesday. Labour’s Scottish leader, Anas Sarwar, who was also pictured alongside striking railworkers, tweeted that the government was to blame for failing to avert industrial action. Starmer’s deputy, Angela Rayner, a former union rep who has regularly appeared on picket lines in the past, also tweeted her support for the RMT strike without attending in person, saying “workers have been left with no choice”. Starmer’s decision to order colleagues to give the picket lines a wide berth has caused irritation among many Labour MPs. Some frontbenchers have even privately suggested that if any of their colleagues were sacked by Starmer as a result, they would then show their anger by attending a picket line themselves. Starmer has been keen to avoid being caricatured by the Conservatives as supportive of the strikes – the most widespread action on the railways for three decades. A Labour source said: “Keir’s Labour party is on the side of the public, and our decisions are driven by that. Not everyone will like it, and we’re prepared for that, but that’s how we’ll prove ourselves as a serious operation, a serious government in waiting, and is how we’ll win elections.” Shadow transport secretary Louise Haigh has repeatedly attacked her opposite number, Grant Shapps, for refusing to sit down with both sides and try to negotiate a deal. The Labour leader has also repeatedly seized on opportunities to demonstrate that he takes a different approach from his predecessor, Jeremy Corbyn, a staunch supporter of union action who was often found on picket lines. During his early months in the post, Starmer’s slogan was “a new leadership”. At prime minister’s questions, Starmer set the rail disruption alongside a series of government failings, including backlogs for renewing passports and cancelled flights. He said the government hadn’t “lifted a finger” to prevent the strikes.