Unite general secretary Sharon Graham says the union is putting the government “on notice” over threats to limit the right to strike.
Transport secretary Grant Shapps announced that ministers are looking at drawing up laws which would make industrial action illegal unless a certain number of staff are working this weekend.
Referring to a pledge in the Conservative manifesto for minimum services during strikes, he said: “We had a pledge in there about minimum service levels.
“If they really got to that point, then minimum service levels would be a way to work towards protecting those freight routes and those sorts of things.”
It comes as more than 40,000 members of the Rail, Maritime and Transport union (RMT) at Network Rail and train operators have been voting on whether to launch a campaign of industrial action over jobs, pay and conditions.
Responding to the latest threats by the government to undermine workers’ right to strike, Graham said: “Unite will confront head-on and by whatever means necessary, any further attacks on the right to strike.
“In Britain we already operate under the most restrictive labour laws in Western Europe. A workers’ right to withdraw their labour is inalienable in any democracy worth its name.
“This is a cynical, authoritarian move designed to protect corporate profits and has been wheeled out to satisfy the needs of short-term factional politics.
“While corporations make billions and ordinary working people suffer, this government chooses to attack the rights of British workers.
“When P&O, a billion-dollar company owned by a foreign dictatorship, brutally sacked 800 British workers, they broke the law. The government’s response was a fine.
“When British workers threaten to defend their living standards in the face of a cost-of-living crisis not of their making, this government threatens to take away their democratic rights.
“We are now forced to put the government on notice. Unite will not sacrifice the protection of our members’ jobs, pay and conditions on the altar of ‘party gate’. If you force our legitimate activities outside of the law, then don’t expect us to play by the rules.
“And Labour now needs to stand up and be counted. I don’t want hard-to-believe promises for the future; we need concrete action now. This is an attack on working people and on the whole of the labour movement. It’s time for the political wing that was founded by and continues to be funded by our members to step up to the plate.”