Thousands of workers at Asda have voted in favour of strike action after a long dispute with the supermarket group over pay and bargaining rights.
Asda says it will try to maintain a normal service in stores
The staff work at Asda's distribution centres around the UK, which could be closed down if action now goes ahead.
The GMB union, which represents about 40% of the staff at the centres, said almost three quarters of those who voted backed a walkout.
Its executive is to meet on Thursday to discuss when action should be taken.
The ballot is linked to the GMB's dispute with Asda over the company's refusal to allow it to negotiate nationally for its staff at all Asda depots.
Asda - whose owner, US supermarket giant Wal-Mart, has long resisted union rights in its home operations - says that local collective bargaining takes place at nine of its 24 UK depots.
The two sides also disagree about profit-related bonus pay.
David Smith of Asda said it would do everything it "lawfully can" to make sure shoppers did not notice any difference at stores.
"It makes us really sad the GMB's threatening strike action when there's already a great offer on the table that could lead to collective bargaining at all our depots, if that's what the majority of our colleagues tell us they want," he added.
"We simply want to give all our people the chance to decide what type of union agreement, if any, they want at their site, not just the minority that are members of the union."
The GMB said it could take legal action against any employment agency which supplies staff to Leeds-based Asda in the event of a strike.
After the ballot announcement, GMB general secretary Paul Kenny said: "GMB members tell us that the company is gearing up and may illegally attempt to use agency labour to do our members jobs.
"Using agency labour during an official trade dispute is illegal and GMB will not allow Asda Wal-Mart to run a coach and horses through UK labour law."
He said any attempt by outside agencies to interfere in "lawful industrial action" by GMB members would be "responded to with full vigour".
GMB officials will meet in Manchester on Thursday to discuss their next step forward.
The union members who were balloted work as HGV drivers and warehouse picker and loaders at 20 Asda Wal-Mart distribution depots.
The depot sites include Bedford, Chepstow, Dartford, Didcot, Erith, Falkirk, Grangemouth, Ince George in Wigan, Lymedale Staffordshire, Lutterworth in Leicestershire, Portbury in Bristol, Skelmersdale, Teesport, Wakefield, and Washington.