Associated Society of Locomotive Engineers and Firemen (ASLEF) general secretary Mick Whelan warned that the move would only cause "longer strife and a different form of action."
While details of the planned legislation are currently vague, they would reportedly require workers in six sectors — the National Health Service (NHS), rail transport, education, fire brigades, border security and nuclear energy — to provide "minimum service levels" of staff.
If trade unions refuse to comply, their action could be declared illegal and striking workers sacked.
Conservative Prime Minister Rishi Sunak said on Wednesday that the right to strike "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."
Whelan warned in a Thursday morning TV interview that "there will be a knock on effect" from any such legislation.
"We're currently — with 11 other trade unions — taking legal action against the last set of laws they put in place, and we would look at doing that in future as well," the railworkers’ leader stressed.
"If the government gets away with what it's doing, we'll be left with an inherently unsafe railway system," he added.
Whelan insisted that similar laws in some European countries had proven "unworkable", and that it was hard to see how workers "legitimately, legally" on strike could be forced to work.
"But we'll look at the detail of those laws if we have to comply with them, we will," he added.
Staff organised by ASLEF and the Rail, Maritime and Transport (RMT) union are currently holding a series of strikes after rejecting a pay offer less than half the current rate of inflation, running at 10.7 per cent in Consumer Price Index (CPI) terms.
Opposition leader Sir Keir Starmer, to whose Labour Party ASLEF is affiliated, took the opportunity later on Thursday to attack the Tories over the plan.
"I don't think this legislation is going to work and I'm pretty sure they've had an assessment that tells them that,” Starmer said in his first speech of the year. “It's likely to make a bad situation worse."
He said Labour would look at the government’s proposals, “but if it's further restrictions, then we will repeal it” when the party is next in power.
“And the reason for that is I do not think that legislation is the way that you bring an end to industrial disputes,” Starmer added. “You have to get in the room and compromise.”
Labour failed to repeal anti-strike laws, passed under former Tory PM Margaret Thatcher, when it was last in government from 1997 to 2010.