The GMB union is warning employment agencies not to provide Asda with temporary staff as cover during a possible strike by its members.
A strike could hit Asda's shelves
A spokeswoman for the GMB confirmed it was writing to job agencies to remind them that it is illegal to provide workers to replace striking staff.
The GMB is balloting staff at Asda distribution centres for strike action in a dispute over union recognition.
Asda said it had no intention of using temporary staff to break any strike.
"The union is trying to fuddle the issue," said an Asda spokesman.
"The fact of the matter is that we use agency staff every single day of the year at our depots, to cover everything from staff leave to big surges in demand such as the current World Cup."
The result of the union ballot will be known on Wednesday.
If the GMB's members across Asda's 21 distribution depots vote in favour of strike action, the walkout is expected to take place over the weekend of the World Cup quarter finals in two weeks' time.
It could result in shortages on the supermarket chain's shelves.
The GMB spokeswoman said the law against employment agencies providing workers to replace striking staff was covered by the Conduct of Employment Agencies and Employment Business Regulations 2003 Act which came into force in 2004.
The union says it has 7,000 members at Asda's distribution centres, but the supermarket claims the figure is closer to 4,500.
Relations between Asda and the unions have been troubled for some time.
In February the retailer was fined £850,000 for breaking new trade union laws by offering a 10% pay rise to 340 workers at a depot in Washington, Tyne & Wear if changes were made to working conditions.
Asda is owned by US retail giant Wal-Mart.