Labour MPs will not back trade unions which stage illegal strikes, deputy party leader Tom Watson has said.
Mr Watson said that while Labour was committed to changing existing trade union legislation, it did not want people to break the law.
His comments came after party leader Jeremy Corbyn repeatedly sidestepped questions as to whether he would support illegal strike action.
Mr Watson said the Unite trade union boss Len McCluskey - who has said he is ready to defy a legal requirement that strikes must be backed by a ballot with a turnout of more than 50% - now accepted Labour MPs could not support illegal industrial action.
"I have actually received a letter from Len McCluskey this week where he says he accepts Labour MPs aren't going to support calls for illegal strike action," he told Sky News's Sunday with Niall Paterson.
"We don't support people breaking the law. We don't want people to break the law, we are democrats.
"We are going to change the law so that trade unionists can have greater rights because we think our current framework of laws is very unfair."
Earlier, appearing on BBC One's The Andrew Marr Show, Mr Corbyn attempted to sidestep questions about whether he backed unions calling illegal strikes.
Repeatedly pressed on whether he would back unions defying the law, he said: "I will be with those workers demanding a decent pay rise."
Mr McCluskey called for the Prime Minister to review the law, saying he would support the 50% threshold if there were workplace ballots rather than "archaic" postal votes.
He told ITV's Peston On Sunday: "I'm urging the Prime Minister not to push workers outside of the law."
He confirmed Mr Watson's comments that he did not expect Labour to back illegal strikes.
Mr McCluskey said: "I don't expect the leadership of the Labour Party - or any Labour MP - to support a call to be outside of the law.
"I know that they are opposed to the current law and they will do all that they can inside Parliament to change this law.
"I'm not looking for that support, I'm not looking to be pushed outside of the law."