Labour MPs who took part in rail strikes will be told a "serious party of Government does not join picket lines", David Lammy has said.
The Shadow Foreign Secretary risked stoking Labour tensions by saying it's not "helpful" for the party to stand on picket lines.
Several frontbenchers defied Keir Starmer's instructions and joined striking RMT workers last week as part of a major dispute over pay and working conditions.
The Labour leader had ordered his top team to stay away from the picket lines in an attempt to avoid Tory efforts to pin the blame for rail strikes onto Mr Starmer's party.
Those who defied his orders are expected to get a talking to from the chief whip and risk potential disciplinary action.
But the decision has sparked anger in some Labour circles.
Mr Lammy told BBC's Sunday Morning: "I suspect the chief whip will be speaking to them next week and making it very clear that a serious party of Government does not join picket lines.
"It's likely there will continue to be disputes over the coming weeks and months - we're in a cost of living crisis and people will no doubt be concerned about rising inflation and where their salaries are as a result of that.
"A serious party of Government, in Government, is in the business of negotiation, sitting down with both those who are striking and management.
"Look I don't think it's helpful to stand on picket lines."
Asked if Mr Starmer has lost control of his own MPs, Mr Lammy said: " Keir Starmer has not lost control of his own MPs.
"Parties that have lost control are parties that lose votes and I think that's the Conservatives and the PM who has lost his back benches."
He said Labour was not divided on the issue and insisted the Tories were trying to stoke a row when Labour is firmly on the side of workers.
Mr Lammy said it wouldn't be "responsible" to support every strike and made it clear he opposed walkouts by BA staff at Heathrow Airport this summer.
"I recognise that there's a cost of living crisis, I recognise that workers are hurting and I recognise they've got a government that's not listening but I do recognise it's a negotiation - get around the negotiation table and fix this problem," he said.
Mr Lammy also disputed reports that the Labour leader has tasked allies with succession planning in case he's fined by Durham Police for drinking a beer and eating a curry at a campaign event in 2021.
The Shadow Foreign Secretary pointed to Labour's recent by-election triumph in Wakefield and added: "Don't break what is working."
Mr Lammy later went further and said he did not support the strikes.
He told TimesRadio: "No, I don't support the rail strikes, because I support the right to strike of course. But you know, it's very sad when any union calls its members out on strike.
"It hurts working people who need to get to work by using the railway. And of course, those within the union are hurt as well. So I absolutely support the right to strike.
"But I'm very sad that it's reached this stage and it's reached this stage because the government's not showing leadership."
Ex-shadow chancellor John McDonnell said he was "disappointed" by Mr Lammy's remarks.
He said: "I think it's an error, and they need to pull back and just do what a Labour MP should do, which is support working people and when they go into disputes like this, support them in every means possible - and that includes picket line attendance."
Andrew Fisher, a former Labour policy chief under Jeremy Corbyn, tweeted: "A serious party of government makes it clear where it stands on the most important issues of the day.
"Do you support workers standing up to protect their pay and jobs, or not David Lammy?"
Unite boss Sharon Graham also criticised Mr Lammy's comments on BA workers.
"Supporting bad bosses is a new low for Labour," she said.
"David Lammy has chosen to launch a direct attack on British Airways workers. This is a group of workers who were savagely attacked by their employer during Covid.
'Fire and rehire' led to thousands of unnecessary job cuts and pay being slashed.
"This dispute is not about a pay rise - it's about restoring money taken out of workers' pockets by an opportunistic employer."
