Hundreds of stores will be opening on Christmas Day this year, with supermarket chains saying more customers than ever are treating it as just another shopping day.

Budgens, Sainsbury's, Safeway, Woolworths and Somerfield are all opening outlets on 25 December because of the demands of city customers.

Budgens is opening 110 of its 233 stores on Christmas Day, mainly smaller Express and Local branches or shops attached to garages. Safeway and Sainsbury's are keeping their bigger branches shut but 51 Safeway stores at petrol stations and six Sainsbury's local shops will be open. Somerfield is opening nine shops in London, Manchester, Birmingham and Newcastle, and Woolworths is opening four branches in London.

All the chains said only volunteer staff would be working. Most of the shops were in areas with large numbers of ethnic- minority staff, who did not celebrate Christmas and were happy to work for premium wage rates, they said.

But Usdaw, the shop workers' union, warned yesterday that the trend would lead to big supermarkets following suit and staff being forced to work on Christmas Day.

"There is a growing trend towards Christmas Day opening, which we are trying to combat. If one of the large supermarket chains decided to open a major store the rest would follow and before long we would have Christmas Day the same as any other day," said Barry Allen, a spokesman.

"They claim that they have enough volunteers from among their ethnic-minority staff but we are moving towards an element of coercion, particularly for managerial staff. It is accepted that emergency-service workers should have to work on Christmas Day, but retailing is not an emergency service."

Lord Davies of Coity, the union's former general secretary, has piloted through the Lords a Bill that would ban big shops from opening on Christmas Day and Mr Allen said it could get Commons approval next year.

Some in the industry believe retailers are unlikely to open big stores because that would be uneconomic.

Rod Alexander, a Budgens spokesman, said smaller stores did extremely good business on 25 December, with many customers making emergency buys while others did their weekly shop.

"I worked Christmas Day one year and I was flabbergasted. It was extraordinarily busy," he said. "I was amazed at how many people treated it as a normal shopping day. Christmas has changed so much in the last 20 or 30 years. For many people, it is not the festival that it was."