The Co-operative Group has become the third supermarket to sign up to a voluntary clampdown on super-strength alcohol sold in cans, as the Department of health races to stop the pledge falling flat with suppliers.

The Co-op Group joined Morrisons and Tesco in signing the pledge to stop selling any carbonated drinks in cans containing more than four units of alcohol by December 2014, though it said it had already met the conditions of the pledge before signing up.

“We have signed the Responsible Can Packaging Pledge as we believe that it, along with the other pledges which comprise the Public Health Responsibility Deal, has the potential to reduce harmful and abusive consumption of alcohol,” a Co-op Group spokeswoman said.

However, the pledge was primarily aimed at suppliers, and the DH has so far failed to persuade all except AB InBev to back the plan.

The drinks giant became the first to sign the pledge in July - agreeing to stop selling 500ml cans of the 9% Tennent’s Super lager and switch ­production to smaller 450ml cans.