The future of thousands of steelworkers' jobs is in doubt after Tata Steel decided to sell its UK assets, including the country's biggest plant at Port Talbot.
Unions attacked the move and Labour leader Jeremy Corbyn urged the Government to intervene as a matter of urgency, possibly by taking a public stake in the industry.
There were calls for the Welsh Assembly to be recalled to discuss the crisis.
Tata made no comment but sources told the Press Association that its board had decided to put its UK assets up for sale.
Union leaders travelled to Mumbai where the Tata board met to discuss the company's loss making UK business, reported to be losing £1 million a day.
They had been hoping Tata would agree to a turnaround plan to keep steelmaking in Port Talbot and other UK plants. The news will affect other Tata sites including Rotherham, Corby and Shotton.
Tata announced over 1,000 job cuts in January, including 750 in Port Talbot. Thousands of steel jobs have been lost in the past year, with companies blaming cheap Chinese imports and high energy costs.
Mr Corbyn said he was "deeply concerned" at the news, adding: "Ministers must act now to protect the steel industry and the core of manufacturing in Britain.
"It is vital that the government intervenes to maintain steel production in Port Talbot, both for the workforce and the wider economy, if necessary by taking a public stake in the industry."
Labour MP Stephen Kinnock, who is with the union delegation in Mumbai, had urged Tata to "hold its nerve " in the face of the problems it was facing.
He was critical of the Government for not sending a minister to lobby the Tata meeting. Business Secretary Sajid Javid is in Australia on an official trip.
The decision came completely out of the blue to union officials.
Dave Hulse, national officer of the GMB union said: "This is absolutely devastating news for all our members, their families and the local communities. Tata has let the whole of the UK steel industry down."
Unite general secretary Len McCluskey said: "This is a very dark day for the proud communities and a proud industry which is now on the verge of extinction in this country."
Roy Rickhuss, general secretary of Community, said: "We travelled to Mumbai to secure a future for steel making in South Wales and we are disappointed that the future remains uncertain, not just for Welsh steelworkers but for thousands more workers in Tata's businesses elsewhere in the UK."