Sir Keir Starmer has said Labour must move from being a “party of protest” to one that can win an election, in order to help working people.

The Labour leader said he supports people’s right to strike, pointing to his pro-bono work as a lawyer representing striking miners.

“Not just sentiment and a photo opp. I backed up my words with action,” he wrote in the Sunday Mirror.

It comes as Sir Keir seeks to contain a row with unions and the left wing of his party over his decision to sack Sam Tarry from the front bench after he gave broadcast interviews from a picket line.

Sir Keir, who previously banned frontbenchers from joining strikers on picket lines, said he fired Mr Tarry as a shadow transport minister after he booked himself on to media programmes without permission and made up policy “on the hoof” in defiance of the party’s “collective responsibility”.

But the move was met with fury by Labour’s left wing and trade unions, with Sharon Graham, the general secretary of Unite, saying she is “aghast” at Sir Keir’s approach to striking workers.

In his article, Sir Keir emphasised his immediate focus of getting his party into power, with a Labour government then able to deliver change for unions and working people.

He wrote: “I completely understand why people are going on strike to secure better pay and better conditions. I support their right to do so.

“When I was a lawyer, I represented striking miners for free. Not just sentiment and a photo opp. I backed up my words with action.

“I am now leading a Labour Party that wants to change lives and give Britain the fresh start it needs.

“That means turning from a party of protest into a party that can win power – then hand that power to working people. I make no apologies for that”.

The Labour leader also lambasted the Conservatives’ record in government.

“Everywhere you look in Britain, you see the toll of 12 years of Tory government,” he said.

“People are working harder but getting less in return”.

He attacked the candidates vying to replace Boris Johnson as prime minister, saying: “Those responsible should hang their heads in shame.

“And as Boris Johnson stooges, that includes Rishi Sunak and Liz Truss. Simply continuity candidates. Unable to offer the change needed.”

Sir Keir said Labour is on the side of people left “angry and scared” by the prospect of skyrocketing energy bills.

“Only in power can Labour deliver the changes that trade unions and working people want to see,” he wrote.

“This isn’t just our agenda, or the trade unions’ agenda. It’s an agenda for the whole country.”

Ms Graham told the Mirror on Saturday: “If I was speaking to Keir right now I would say to him: which side are you on? Because the reality is, if I closed my eyes, sometimes I wouldn’t know whether it was the Labour party or the Tories who were speaking.

“This is one of the biggest crises that workers are facing – we are trying to defend them with every fibre of our being and the party who is supposed to be echoing that in Parliament is doing the exact opposite.

“I’m very disappointed – aghast, quite frankly – and I think it’s something Labour is going to have to think seriously about.”

Mr Tarry said he “absolutely” still thinks Sir Keir is the best person to become prime minister.

Joining another picket line at Paddington station in London on Saturday, he called for a “fundamental recalibration” of the Labour Party’s relationship with the trade union movement and said it needed to demonstrate it was on their side.