Tata Steel is to sell its UK assets, placing thousands of British workers at risk of losing their jobs.
Labour leader Jeremy Corbyn said he was "deeply concerned" by the news from Tata and called on the Government to intervene.
The Mumbai-based firm confirmed that it intends to sell its entire UK steelmaking operation on Tuesday evening.
In a statement, Tata noted that trading conditions in the UK and Europe have "rapidly deteriorated."
"These factors are likely to continue into the future and have significantly impacted the long term competitive position of the UK operations in spite of several initiatives undertaken by the management and the workers of the business in recent years," the statement said.
Complaining of "asset impairment" of more than £2 billion in the last five years, the Tata board concluded that a plan aimed to save UK plants was unaffordable.
"Following the strategic view taken by the Tata Steel Board regarding the UK business, it has advised the board of its European holding company ie Tata Steel Europe, to explore all options for portfolio restructuring including the potential divestment of Tata Steel UK, in whole or in parts."
Union leaders had flown to India hoping to convince the company to keep its Port Talbot and other plants in operation.
Reacting to the news, Britain’s largest union Unite demanded the UK government honour promises to secure a future for the British steel industry.
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."
“This news will leave workers and families not just in Port Talbot but in Rotherham and Stocksbridge reeling," he added. "Steel is in the veins of these communities.”
“They will feel a grim sense of betrayal by this decision, because they know that given half a chance they can make Port Talbot and sites across the UK profitable and successful,” McCluskey said.
On Tuesday, the UK and Welsh governments released a joint statement that said they "remain committed to working with Tata and the unions on a long-term sustainable future for British steel making."
"Both the UK and Welsh governments are working tirelessly to look at all viable options to keep a strong British steel industry at the heart of our manufacturing base," the statement said.
Corbyn called for ministers to "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," the Labour leader said.
In January, Tata announced 1,000 job cuts in the UK. The company has blamed cheap Chinese imports for the industry’s struggles.