Keir Starmer is expected to discipline at least five Labour frontbenchers who defied his orders and appeared on RMT picket lines on Tuesday in solidarity with striking railworkers. On the first of three days of industrial action, the Labour leader had instructed his team not to appear alongside striking workers, in order to show “leadership”, amid fears of Labour being portrayed by the Tories as responsible for the RMT’s walkout. But Starmer now faces a test of his own leadership, after five frontbenchers tweeted pictures of themselves alongside RMT strikers. They included the shadow minister Alex Sobel, Labour whip Nav Mishra and three parliamentary private secretaries. Starmer is not expected to decide how to discipline the errant MPs until after the strike is over this weekend, but his allies suggested he was not minded to be lenient. Several frontbenchers privately expressed exasperation at what they saw as an unnecessary internal dispute that has thrown the spotlight on Labour’s divisions. One shadow cabinet member, who predicted that Starmer would pull back from sacking the rebel MPs, described his decision to ban frontbenchers from picket lines as “dumb”. Labour’s Scottish leader, Anas Sarwar, disregarded Starmer’s strictures and visited his local RMT picket line in Glasgow, while some shadow ministers found creative ways of declaring their support for the striking workers without breaking their leader’s ban on showing up in person. Starmer’s deputy, Angela Rayner, who was a union rep before entering parliament and has frequently appeared on picket lines, posted what appeared to be a very carefully worded tweet, saying “workers have been left with no choice”. The shadow transport minister, Sam Tarry, tweeted a picture of himself at another recent RMT protest, with the message, “workers don’t take the decision to strike lightly … Solidarity with workers risking their livelihoods for a better tomorrow.” The shadow minister for homelessness, Sarah Owen, shared the link to the RMT’s strike fund, tweeting, “people never decide on industrial action lightly but I fully support the right to do so”. Starmer did not make a public appearance on Tuesday, but a Labour source said he stood by his decision to ban frontbenchers from attending, which reflected his confidence. “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,” the source said. One loyal shadow cabinet member said Starmer’s stance reflected the need to show that a Labour government would be able to act as an honest broker in future industrial disputes, rather than taking a side – though they still bemoaned the “self-inflicted” row that has ensued. A snap poll by Savanta ComRes on Tuesday showed 58% of the public believed the rail strike was justified, once given some background – including the fact that Network Rail had said modernisation was necessary, but the RMT argued the 2,500 planned job cuts could make accidents more likely. Two-thirds of those polled thought the government had not done enough to prevent the strikes, and 49% said the same of the RMT. Many Labour backbenchers also attended picket lines on Tuesday, most of them members of the leftwing Socialist Campaign Group, including the former shadow chancellor, John McDonnell. A spokesperson for the leftwing campaign group Momentum said: “The Labour party was founded to represent the interests of workers. But under Keir Starmer’s leadership, the party has lost its way.” As Starmer wrestled with his party’s stance on the strikes, one senior Conservative backbencher said he was lukewarm over the government’s attempts to paint the opposition as supporting the rail strike. “I’m not personally convinced that calling it a ‘Labour strike’ and all the rest of it really achieves much, but at the same time I don’t think what Grant Shapps says makes a lot of difference,” he said. “The issue is that if the strike badly impacts people they will blame the government anyway.” Meanwhile, ministers are expected on Thursday to put before parliament legislation that will allow companies to use temporary workers to replace striking staff. Downing Street sees the controversial step, condemned by the Trades Union Congress as taking anti-union action beyond even that introduced by Margaret Thatcher, as a key response to the rail strike and the threat of action by other public sector workers – even though it remains unclear how agency staff could replace skilled employees. The government’s bullish approach towards the strike was emphasised by Boris Johnson’s comments at the start of Tuesday’s weekly cabinet meeting, in which he condemned “union barons” from the RMT. The walkout was “wrong and unnecessary”, the prime minister added.