Unite is pressing Tata Steel to give cast iron guarantee to end the "rollercoaster of uncertainty" for British Steel workers following the group's decision to halt the sale of its UK assets.
Tata said late last night that it had paused the sale of its biggest UK assets as it entered talks over a joint venture with German rival Thyssenkrupp. The Brexit vote and the British Steel pension fund which has a deficit of £700m were both factors.
Unite national officer, Harish Patel, said the union wants "cast iron guarantees over its intentions for Port Talbot and its strips business. The rollercoaster of uncertainty the world class workforce has been on must end."
"With talk of a tie up with ThyssenKrupp, this announcement cannot result in the managed decline or Port Talbot while capacity is shipped elsewhere."
Meanwhile, trade body EEF added that the announcement shouldn't stop the government from helping the wider industry, which is in even greater peril following Britain's decision to leave the European Union.
Gareth Stace, director of UK Steel, said: "There are some very simple steps that government can take in the near term to provide support, such as cutting energy and other business costs."
"In the mid-term it can protect our interests during Brexit negotiations by ensuring continued access to the EU market. These actions will help secure our future and ensure that British steel making is able to provide post-Brexit Britain with much-needed security of supply."
Liberty House, which was one of several potential bidders for Tata's UK business, said that it feared the decision would bring about more longer-term uncertainty for the UK steel industry.
"This is sad and frustrating news for the UK steel industry because the sector needs a fresh start," a spokesman for Liberty House stressed.