Following a number of customers reporting that they tried to buy pork or alcohol from a Muslim shop assistant in central London, only to be told that they would have to use another till because of the cashier's religion, a Facebook campaign has called for the boycott of the British brand while the official M&S Facebook page was hit by vitriolic messages from previously loyal customers.

"Where we have an employee whose religious beliefs restrict food or drink they can handle, we work closely with our member of staff to place them in suitable role, such as in our clothing department or bakery in foods," said M&S in a statement, while admitting that on this occasion it had to "regret" that it had not followed its own guidelines.

"We regret that in the case highlighted today we were not following our own internal policy."

The spokesman said the policy applied throughout its 700-plus stores and to other religions as well. Christians who did not want to work on Sundays and religious Jews who chose not to work on Saturdays would also be excused.

However, a new campaign group called Boycott Marks and Spencer has several messages that accuse the retailer of creating "division and hatred within our communities".

One customer wrote: "M&S if you read this you have gone over the top. If you have Christian workers who wish to refuse the sale of ladies garments to male homosexuals or men's trousers to lesbians, I do hope you will equally stand by those workers' religious or personal beliefs.

"Or perhaps if an atheist worker notices a customer purchasing hot cross buns during Easter and wishes to refuse sale to that particular customer based on beliefs that conflict with their own, I do hope you will not punish such workers for their actions surrounding refusal of sale...

"If not then I would like you to know that my family and I shall no longer purchase any goods from your company due to the implementation of this 'one rule system' that creates further division and hatred within our communities.

Meanwhile, another customer wrote: "If M&S go ahead with this they are going to lose so many loyal customers... if they do not want to serve people with pork or alcohol they shouldn't work in the food hall... simple!"

While another wrote: "I shan't be shopping in M&S anymore. The quintessentially British retailer bows down to Muslim beliefs. And in turn alienates the majority of Christian and non-religious customers. Outrageous."