Staff at Hall's of Broxburn, in West Lothian, were reeling after the bombshell announcement by Dutch owner Vion Foods, which said the factory is losing almost £80,000 a day.
Union officials reacted angrily to the news, which they claimed was completely unexpected.
It was also described as a "body blow" to the Scottish pig farming industry.
The 80-year-old firm's pork sausage and haggis products are stocked by major supermarkets, and it handles around 8000 pigs a week.
The Scottish Government immediately set up a task force, which met for the first time last night, to try to prevent one of the biggest factory closures in Scotland since Motorola shut its Livingston plant in 2001, with the loss of more than 3000 jobs.
Vion blames its losses on "extremely challenging" conditions in the UK meat industry, and has launched a 90-day consultation on the future of the plant.
Stewart Forrest, USDAW'S deputy Scottish divisional officer, whose union is part of the task force, said: "This is absolutely catastrophic news for everyone who works at Vion Hall's and for Broxburn and West Lothian. Our members are understandably shocked and extremely angry at the announcement.
"A 90-day consultation period will start, but the first question we want answered is why the company has waited until this eleventh hour before engaging with its employees and their union.
"USDAW was given no indication of the true seriousness of the situation, and that is extremely regrettable and, in my view, totally unacceptable."
Vion UK's chairman Peter Barr said: "This is an extremely sad day and one we have strenuously tried to avoid for the past four years, but the huge losses being incurred mean we believe we have no alternative. Every possible step has been taken to secure the future of the business but we are currently losing £79,000 per day at the site, which is clearly unsustainable.
"There is significant over-capacity in the UK meat industry and market conditions are extremely challenging."
Hall's has been making meat products since 1932 and is one of Scotland's most recognised brands. It holds a royal warrant and claims on its website to be Britain's biggest-selling haggis brand.
Finance Secretary John Swinney, who said the news was "extremely disappointing", held talks and offered his support to Vion and its 1150 permanent and 595 agency staff.
He also took part in a meeting of the task force, which included USDAW, West Lothian Council, local MPs and MSPs, and various business groups and Government training and jobs agencies.
Speaking after the first meeting in Broxburn last night, Mr Swinney said: "The task force agreed that its key focus is maintaining continuity of business at Hall's of Broxburn and we are entirely committed to that exercise.
"We have drawn together all of the relevant Government agencies, local agencies and political representatives to focus on ensuring we deliver a positive economic future for the workforce and community of Broxburn. The task force has had a positive start and will take forward a number of practical actions before re-convening on July 30."
NFU Scotland said it is a blow for the
plant's many suppliers. The union's Pigs Working Group chairman, Philip Sleigh, said: "News that Vion's Broxburn plant is to undergo a three-month review pending closure will stun and disappoint pig producers and the wider food industry.
"The plant, which at present processes 8000 pigs per week, also houses a huge processing facility, and the news is inevitably a savage blow for Vion's Broxburn employees, suppliers and customers."
He said the NFUS will work with its members, Vion and the Government to ensure "as good an outcome as possible" and called on retailers to honour existing contracts with the union's pig co-operatives.
Jim McLaren, chairman of Quality Meat Scotland, said the news was a "body blow" for the country's meat industry. Vion announced in September the creation of 250 jobs at the site, including an apprenticeship scheme that attracted Scottish Government grants of up to £2m. The firm said the funds were never drawn.
West Lothian Council is to hold a special meeting on Tuesday to discuss the situation.