About 250 jobs will be lost at the Walkers snacks factory in Swansea after the company confirmed it will close.
Walkers want a location closer to the Midlands and north west
The firm is transferring the work to other sites, including Peterlee, County Durham, and Skelmersdale, Lancashire.
Closure will be phased in over six months, with all staff offered jobs in other plants. Walkers said it was a "difficult decision," but would mean more efficient distribution.
Local politicians said the announcement was a severe blow to the region.
The plant produces a wide range of Walkers products, including Wotsits, Monster Munch and Bacon Fries.
Walkers president Martin Glenn said: "In today's increasingly competitive environment, we have to operate as efficiently as possible to ensure the continued success of our business.
"With this in mind we have taken the decision to move production from Swansea to other Walkers sites that are closer to our distribution centres.
"This will allow us to make cost savings on the snacks that are currently made in Swansea and enable us to distribute them more efficiently to retailers.
"Although we are closing the Swansea plant, we're hoping that all of the people who work there will continue to work with us.
"Everyone who works at the plant will be offered jobs at our other sites and we will be talking to people individually about that over the next few weeks.
"Everyone will receive our full support and assistance whether they choose to continue to work with us, which we hope they will, or if they decide to look for alternative employment in the Swansea area".
The factory is a listed building and one of the landmarks on the western approach to Swansea because of its square clock tower. It was formerly the home of Smiths Crisps.
Before the announcement some workers said they were puzzled because the products were as popular as ever with customers, and there had been a lot of recent investment in new machinery.
Swansea council called the closure a 'bitter blow'
Swansea East MP Sian James said the plant was going, "even though it produces some of Walkers' best-known and best-selling products".
"I will be doing everything I can, along with my colleagues in the National Assembly for Wales and trade unions, to support the employees affected by this decision today," she said.
Chris Holley, leader of Swansea Council, said closure was "a very bitter blow for the whole region".
Mr Holley said he first learned about the expected announcement from BBC Wales, and it was a "severe shock."
"We'll contact the Welsh Development Agency and Welsh Assembly Government and obviously we'll work with all these agencies to try to lessen the impact on the local people and jobs market."
South West Wales Plaid Cymru AM Dai Lloyd said: "This decision is a devastating blow to the local economy, especially as it comes only a relatively short time after the loss of over 400 jobs at Dewhirst which was only 200 yards away.
"As the local doctor who used to be responsible for this factory I can personally sympathise with the workers who have received this bad news today."
Walkers, which has been advertised for several years by BBC presenter and former England football captain Gary Lineker, was founded in Leicester shortly after World War II.
The company employs 4,000 people at 15 sites and according to its website, 400 million packets of snacks are produced in Swansea each year.