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 party’s leader said he supports the right to strike, pointing to his pro-bono work as a lawyer representing striking miners instead of “just sentiment and a photo op”.

Rail services around Great Britain were severely disrupted on Saturday by the most widespread strike by train drivers since rail privatisation in 1996. Members of the Aslef union stopped work for 24 hours at seven train operators.

It comes as Starmer seeks to defuse a row with unions and the left of his party over his decision to sack Sam Tarry as transport minister after he gave broadcast interviews from a picket line.

Starmer, who previously banned frontbenchers from joining strikers on picket lines, said he fired Tarry after he booked himself on media programmes without permission and made up policy “on the hoof”, in defiance of the party’s “collective responsibility”.

In an article in the Sunday Mirror, Starmer emphasised his immediate focus on 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 op. 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”.

In an interview with the Observer, Sharon Graham, the general secretary of Unite, said Labour is becoming “irrelevant to workers” and it is now hard to justify handing the party millions in funding.

Graham said she felt the party’s leadership was, in effect, “sticking two fingers up” at workers with its response to strike action and its abandonment of pledges to renationalise public utilities.