Tesco is facing Britain's largest ever equal pay claim and a possible compensation bill of up to four billion pounds ($5.6bn), according to a law firm which has begun legal proceedings.

Tesco is Britain's biggest retailer and its largest private sector employer with a staff of over 310,000.

Law firm Leigh Day said that thousands of Tesco employees working in its predominantly male-dominated distribution centres are paid considerably more than the largely female-staffed Tesco stores.

It said distribution centre staff may earn in excess of $15.40 an hour while the most common grade for store staff sees them receive around $11.12 an hour.

This disparity could see a full-time distribution worker on the same hours earning over $140 a week, or $7,000 a year, more than female based store staff.

Leigh Day said it has already started submitting claims on behalf of its clients.

It said the underpayment could apply to in excess of 200,000 Tesco employees and with estimated pay shortfalls that could reach $28,000 the final bill for Tesco could be as high as $5.56bn.

"We are unable to comment on a claim that we have not received," said a Tesco spokesman.

But he added, "Tesco has always been a place for people to get on in their career, regardless of their gender, background or education, and we work hard to make sure all our colleagues are paid fairly and equally for the jobs they do."