A Labour government would repeal any new anti-strike laws introduced by the Tories, Sir Keir Starmer confirmed on Thursday.
The Labour leader said legislation to enforce "minimum service levels" would be reversed if his party won the next general election.
Prime Minister Rishi Sunak is finalising plans that would require a proportion of union members in key public services to continue working during industrial action.
Strikes could be deemed illegal if members refuse to provide this and unions that do not to cooperate could face being sued.
The laws are likely to hit the railways, health, fire and education sectors, as well as some energy industries.
Nurses and ambulance workers who are due to strike later this month already agree minimum safe levels of staffing with employers before walkouts.
During a new year speech in Stratford, east London Sir Keir said: "I don't think this legislation is going to work and I'm pretty sure they've had an assessment that tells them that. It's likely to make a bad situation worse."
He added that if the proposals were brought forward then "then we will repeal it" if the party won the next general election.
"The reason for that is I do not think that legislation is the way that you bring an end to industrial disputes," he said.
"You have to get in the room and compromise.
"You can't legislate your way out of 13 years of failure. The government's all over the show. Will we repeal it? Yes, we will."
Labour's deputy leader Angela Rayner described the proposals as "offensive, unserious and unworkable".
It comes as rail services were crippled by the walkout Aslef union members at 15 rail companies on Thursday, in a long-running dispute over pay.
There were no services on Thursday morning on Southeastern, Thameslink, Great Northern or Avanti West Coast, while South Western Railway and Greater Anglia limited trains into Waterloo and Liverpool Street.
Victoria, normally one of the busiest stations in the county, was closed, with no Southern or Gatwick Express trains running. Passengers there described the situation as a “nightmare” and “bad for Britain”.
It follows industrial action by the RMT on Tuesday and Wednesday.
Aslef boss Mick Whelan told Sky News "there will be a knock on effect" from any new laws ministers pass around striking.
"We're currently - with 11 other trade unions - taking legal action against the last set of lawsc they put in place," he said.
"We would look at doing that in future as well."
Yesterday, in his new year 's speech Mr Sunak hinted his government's new approach to strikes would be unveiled "in the coming days".
He said the while workers have a right to industrial action "that has to be balanced with the right of the British public to go about their lives without suffering undue disruption in the way we've seen recently".
