Marks & Spencer’s ethical trading tie-up with development charity Oxfam has increased general merchandise basket sizes at the retailer.

The scheme, which rewards donors of M&S clothing to Oxfam with£5 vouchers that can be redeemed when spending£35 or more, has outperformed M&S’s expectations.

M&S head of corporate responsibility Mike Barry highlighted the partnership’s success at a retail sustainability forum organised by PricewaterhouseCoopers.

A source reported that Barry said M&S’s average in-store transaction was “in the low 20s”, but that those redeeming vouchers spent more than£40. The incremental rise generated by the voucher was therefore about£20. About 234,000 people have donated M&S clothing to Oxfam since January, it was revealed last month.

No comment was available from M&S or PwC on the voucher, but the success of the deal – launched to coincide with the first anniversary of the retailer’s Plan A programme – highlights the increasing importance of ethical and environmental responsibility for retailers and the potential for it to benefit sales.

PwC UK retail and consumer leader Mark Hudson said: “The best-run companies will become the most sustainable and ultimately the most successful. Those moving first and fastest are creating resilient models for the long term, changing the rules of the game and generating commercial advantage.”

Corporate social responsibility made headlines again this week when Primark ditched three Indian factories that had breached its ethical standards. The fashion retailer said it would appoint a “reputable NGO” in southern India to “act as its eyes and ears on the ground” and help ensure rules are followed.