Owners Vion Food UK told workers that management were continuing "detailed discussions" with interested parties but that a phased shutdown will start later this month if a "sustainable solution" cannot be found.
Vion has already turned down an offer from the Scottish government to buy and lease back the factory, saying that accepting public money would only delay any potential decision regarding closure.
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 "stringing everyone along", knocking back viable options, putting its "self-interest ahead of the workforce" and being "intent on closing the plant no matter what".
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 has been unable to stem its "unsustainable losses", he said. The plant's layout is "outdated and inefficient" and there is an "over-capacity in the marketplace" for its produce.
"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 will begin later this month, with an anticipated full closure by February.
Finance Secretary John Swinney said the Scottish Government and its agencies have done all they can to secure the future of the plant, and are still working to secure a buyer.
"All possible support will be made available to staff. 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," he said.
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 going forward."
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 fact that staff have recently been asked to work overtime demonstrates that there is still a market for its produce. "The company have chosen to put their self-interest ahead of that of the workforce," she said.
Lothian Labour MSP Neil Findlay said: "I am extremely angry at the behaviour and attitude of Hall's and their parent company Vion. It became clear during the consultation process that they were intent in closing the plant no matter what and unfortunately this has now been confirmed."
Livingston Labour MP Graeme Morrice said: "I am very angry at this decision. It seems to confirm that Vion's senior management have simply been stringing everyone along for the last three months, particularly after it rejected the very generous assistance package offered by the Scottish Government."
The Scottish Trades Union Congress (STUC) pinned the blame for the closure on prolonged "mistakes of management", while the local council leader described the announcement as "a betrayal".
STUC 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.
"The STUC would urge Vion to continue working with Usdaw, the Scottish Government, local authority and other stakeholders to ensure that all remaining options are pursued with extreme vigour."
West Lothian Council leader John McGinty said: "We are deeply disappointed in the decision by Vion. We see this as a betrayal of the hard-working and loyal workforce.
"The taskforce provided every opportunity to the company to discuss and explore the options, including an attractive public sector offer which would have led to significant investment at the plant in Broxburn.
"Regardless of the speculation about the outcome of the 90-day consultation, the news today from Vion will have been a shock for the workforce. It is difficult to overstate the likely devastating impact of the closure of Hall's on individual employees, their families and communities, and it is made worse by the current economic conditions."
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