Food group Vion, owner of Hall's of Broxburn, said it had been "unable to identify any viable alternative to the closure of the plant" and that it may be closed within four months.
Vion UK chairman Peter Barr said the plant would require enormous investment to match "larger and more competitive plants across Europe".
But politicians accused Vion of knocking back viable options and putting self-interest ahead of the workforce.
Mr Barr said: "We have done everything in our power to avoid having to take this extremely regrettable action, but we have been unable to identify any viable alternative to the closure of the plant.
"Unfortunately, the unsustainable losses which the plant continues to suffer, combined with the challenging economic conditions across all food sectors, have left us with no alternative."
The tens of millions of pounds invested in the site over the years have been unable to stem its unsustainable losses, he said.
He added: "While some efficiency improvements have been identified, they are not nearly enough to bridge the very considerable losses which Hall's continues to incur. We explored the feasibility of undertaking a major overhaul of the site. However, the investment required to deliver a facility capable of competing with much larger and more competitive plants across Europe would have been enormous. In the current environment, that level of expenditure is not an option."
Vion is still working to find a potential new owner, but Mr Barr warned that if a solution cannot be identified, a phased closure of the site would begin later this month, with an expected full closure by February.
Finance Secretary John Swinney said the Scottish Government and its agencies had done all they could to secure the future of the plant, and were still working to secure a buyer.
"All possible support will be made available to staff," he said. "Teams have already been on site and resources are in place to help people who are looking at their options for the future. We will also work to address the serious potential impact on the local area."
Rural Affairs Secretary Richard Lochhead said: "With around 65% of the pigs processed at Hall's coming from Scottish farms, this difficult news clearly marks an unsettling time for Scotland's pig industry.
"However, I can assure pig farmers that Scottish pork remains an important part of our food and drink offering."
Broxburn MSP Fiona Hyslop, also a Cabinet minister, said she was "extremely frustrated".
She said: "This is a dark day for Broxburn. Frankly, Vion have a lot to answer for. They have a loyal, skilled and experienced workforce here which they have abandoned, and they have knocked back every effort that has been made by the Scottish Government and others to find a solution. This did not have to happen. The company have chosen to put their self-interest ahead of that of the workforce."
The Scottish Trades Union Congress (STUC) pinned the blame for the closure on management mistakes. General secretary Grahame Smith said: "The STUC is stunned and angry at the news that Vion is to proceed with closing the Hall's of Broxburn plant. Once again, a skilled and productive workforce is paying for the mistakes of management over an extended period. Job losses on this scale will be extremely difficult to bear in an area of the country that has suffered particularly badly through the current economic slump."
West Lothian Council leader John McGinty said: "We see this as a betrayal of the hard-working and loyal workforce.
The Usdaw union said its members at the Hall's site had been left devastated by the developments. Deputy divisional officer Stewart Forrest said: "Many workers have been at Hall's for years, with many families having different generations employed there. The huge workforce at Hall's means the impact of any closure will be felt way beyond the locality."
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