The Government has been accused of "fast talking but slow delivery" in helping the crisis-hit steel industry as workers are braced for more job losses.
Tata is expected to announce around 1,000 job cuts at plants including Port Talbot and Llanwern in South Wales, dealing a huge blow to the industry and the Welsh economy.
Thousands of job losses were announced last year, with companies blaming high energy costs and cheap Chinese imports.
The Community union said it will "vigorously" challenge the company's proposals.
General secretary Roy Rickhuss said: "Today's announcement is no reflection of the skills and commitment of the Tata Steel workforce, which has been breaking production records over the past year.
"Rather, it is yet another chapter of the UK's ongoing steel crisis and the lack of a proper government response.
"This industry needs meaningful action from the UK Government, which up to now has been characterised by fast talking but slow delivery, despite persistent warnings from Community that delays in implementing support for steel would have an impact on jobs.
"Even now, promised compensation for energy intensive industries is yet to be received. The UK Government must step up and work with trade unions and businesses to ensure this industry exists for generations to come.
"The dumping of cheap Chinese steel is one of the biggest causes of this crisis, yet the UK Government remains a cheerleader for China and their bid for 'market economy status', which would decimate what's left of our steel industry. This cannot be allowed to happen.
"As well as government support, it is absolutely vital that Tata Steel make clear their long-term commitment to steel-making in the UK. The workforce has made significant sacrifices in recent years, on the assurance that jobs would be protected.
"However, rather than delivering on this promise, Tata have simply continued to slash jobs. Tata Steel must now come clean about their long-term commitment to the UK. Community, as the steelworkers' union, stands ready to play our part in securing a future for the industry."
Alan Coombs, a Port Talbot steelworker and president of Community, said: "Today's news is a tragic reminder of the UK's ongoing steel crisis. Here in Port Talbot we make some of the world's best steel, but cheap Chinese imports and high energy costs are crippling our industry.
"This announcement will affect everyone here - steelworkers, engineers and office staff. As the steelworkers' union, Community is ready to work with Tata Steel to ensure that steel-making remains at the heart of Port Talbot for many years to come. However, we will not accept anything that threatens the health and safety of workers or the integrity of the plant.
"Port Talbot is a town built on steel - my father and grandfathers worked here - but we risk losing these jobs forever without more action from Government to create a level playing field on which we can compete.
"Thousands of other families here in Port Talbot rely on the steelworks, it is a source of pride and employment for our town, we simply cannot afford to lose it."
Many contractors and service firms rely on the Port Talbot plant, so direct job cuts will have a knock-on effect across the region.
The Government responded to the crisis by holding a summit last year and pressing the European Union to help high energy using firms.
A Business Department spokesman said: "While this remains unconfirmed by the company, these reports are concerning and we are monitoring the situation closely. The Government continues to engage closely with Tata on how we can help during this difficult period for the sector.
"The Government has taken clear action to help the industry, through cutting energy costs, taking action on imports, government procurement and EU emissions regulations, meeting key steel industry asks."