Jeremy Corbyn has refused to rule out supporting illegal strikes by union-backed workers, but the Unite general secretary, Len McCluskey, has said he would not expect Labour politicians to support industrial action outside the law.

The Labour leader said he viewed the Conservatives’ 2016 Trade Union Act as “really unfair” and would back workers fighting for a fair pay rise.

The change in the law makes strikes illegal unless at least half of eligible union members take part in the ballot, with public services such as fire brigades and teachers barred from striking unless 40% of union members vote in favour.

“This law is something that is really unfair,” Corbyn said. “No MP, very few MPs, get more than 50% of their electorate. I think I was one of the very few who does.”

Asked on the BBC’s The Andrew Marr Show on Sunday whether he would support workers going on strike regardless of the law, Corbyn refused to rule it out. “The clear answer is this: I support the campaign for a decent pay level in the public sector,” he said.

“I will ensure that a Labour government repeal the existing trade union law and bring us in line with the international labour organisational conditions.”

Quick Guide Labour conference highlights Show From Miliband to Brexit Sunday

The primary business in the main conference hall is entitled “protecting our communities", and the session ends with a report on June’s election. On the conference fringe, Ed Miliband makes a relatively rare appearance, discussing the economy and austerity. Monday Highlight in the conference hall is the setpiece speech by John McDonnell, the shadow chancellor. For an insight into Labour thoughts on Brexit, a fringe event in the afternoon promises Keir Starmer, Hilary Benn and Yvette Cooper, and is grandly called “How to handle Brexit.” Tuesday Business begins with constitutional changes and Labour’s NEC – which could include moves to allow a would-be successor to Corbyn to stand with the support of relatively few MPs. In the afternoon Tom Watson, deputy leader, will makes his speech, expected to be scrupulously loyal to his leader. Wednesday The conference ends with a lunchtime speech from Corbyn, who arrives in Brighton bolstered by the election result.

Corbyn was asked again whether he would rule out joining striking workers on the picket line if the strike contravened the statute. “I will be with those workers demanding a decent pay rise,” he said.

McCluskey called on Theresa May to reach a compromise over strike ballots, saying he would support the 50% turnout threshold if there were secure workplace ballots instead of postal ballots.

“I’m not looking to be pushed outside the law and I want to appeal to the prime minister. Don’t push ordinary working people outside the law,” he told ITV’s Peston on Sunday. “No one wants to step outside the law, least of all me.”

The union leader said he did not expect Labour or any of its MPs to back illegal strike action as he knew they supported the aims of striking workers. “I am not looking for that support,” he said.

Earlier on the show, the shadow chancellor, John McDonnell, said he believed unions could reach the 50% threshold for strikes and take industrial action within the law. “You’ve got a lack of confidence in the working class. I haven’t,” he said.

“You watch what will happen this coming winter, these strikes have been provoked by this current government and as a result of really pay being below inflation, wages being cut; you know we’ve got nurses going to food banks because their pay has been cut by 14% since these Tories have been in power. No wonder people are angry – I am.”

Tom Watson, the deputy leader, gave a different answer from both Corbyn and McDonnell to the question of whether Labour would back illegal strikes.

“We don’t want people to break the law,” he told Sky’s Niall Paterson on Sunday. “We are democrats but we’re going to change the law so that trade unionists can have greater rights because we think our current trade union laws are very unfair, particularly the last Trade Union Act, which we campaigned against in parliament, and the way to deal with this is to campaign against the government to change their own laws and if they don’t change them, elect a Labour government and we’ll change the law to make sure that our trade union laws are fair.”