The Chevron refinery is the largest private employer in Pembrokeshire
Chevron says it is looking for a buyer for its refinery in Pembrokeshire which employs around 1,400 people.
The US-based company said it wanted to gauge interest in the Pembroke site but said it would not close down.
It processes 220,000 barrels of crude oil a day and is estimated to be worth £1.5m a week to the county's economy.
MP Nick Ainger said managers had told him Chevron planned to invest over £50m at the site and he was hopeful the refinery had "a real future".
Chevron said its intention to look for bids for some of its operations in Europe was part of its global restructuring.
In a statement it said: "To confirm, this process is to determine interest in our operations in Europe.
"No decision has been made to sell any asset at this time.
"We have no plans to idle any of our refineries.
"Our continuing focus is on operating Pembroke refinery in a safe, reliable and environmentally responsible manner."
The union GMB said it was "essential" that the refinery, at which around 600 staff and 800 contractors work, remains open.
Regional officer Jeff Beck said: "This refinery is a huge employer in west Wales. It is essential that the UK and Welsh governments ensure that who ever buys this will pledge to keep it open.
"We can not have another Corus situation in west Wales. GMB will be seeking talks with all concerned to ensure that employment is safeguarded."
Pembrokeshire council said it was "seeking more information and clarification" from the company.
Mr Ainger, MP for Carmarthen West and South Pembrokeshire, said managers had told him the company planned to invest $80m (£53m) over the next two years.
Business as usual
"To me that indicates that Chevron view this as very much a going concern and want to make it an attractive proposition for a bidder," he said.
"They are telling me there will be no lay-offs planned at all.
"They are continuing with apprenticeships, so it's business as usual - that's what the local management are telling me.
"I am hopeful that the refinery has a real future."
Wales Deputy First Minister Ieuan Wyn Jones said he spoke to Chevron this morning and intends meeting the company to discuss its plans.
"They have already told us that no decision has been taken to sell their Pembroke site, but are undertaking an exercise to identify possible buyers for the refinery," he added.
"This process could take at least two years during which time the refinery would remain operational.
"We understand that planned investments at the plant and the apprenticeship programme will continue."
Shadow Secretary of State for Wales Cheryl Gillan MP said it was "extremely worrying news".
"The company must give every assurance to the local workforce that their jobs will not be put at risk," she added.
"I am seeking an assurance from the UK Government that it is in discussions with the firm about the implications of this decision locally and nationally."
Nick McGregor, an oil industry analyst at Redmayne Bentley, told BBC Radio Wales he believed if Chevron could get a creditable price they would sell the plant.
"I think Chevron will certainly look for a reasonable period of time for a buyer but I don't think they are going to sustain their ownership of it in the long term," he warned.
"If they can't find a buyer then I would be quite concerned for the long-term future of Pembroke.
"I think that the only consolation is the fact that there will be a political will to continue refining a reasonable amount in this country and some of the other operations in the UK are also being looked at by their owners."
The refinery, which first opened in 1964, benefited from an £84m upgrade in 2005.