Peter Marks, the chief executive of The Co-operative Group, announced his retirement this morning. Marks, who has been at the helm for the past six years, and has worked in the wider co-op movement for an impressive 45 years, steps down in May next year.

The news raised eyebrows in the City, coming just weeks after Marks shook hands on a landmark deal to acquire 632 Lloyds branches - establishing The Co-operative Bank a major player in the banking sector, with a network of almost 1,000 branches and 11 million customers.

But speaking to The Grocer this morning, Marks insisted his retirement had been “planned for some time” and he hadn’t wanted to disrupt the Lloyds deal.

The transformational banking bargain won’t be Marks’ only legacy when he calls it a day next May. The society has been transformed since he became CEO in 2007 following the merger of the UK’s largest two co-operative societies, The Co-op Group and United Co-operatives.

Marks has long been the most vocal supporter of a unified co-op movement and the merger went some way to show other independent co-ops that bigger can be better. It created a massive 50% uplift in profits for the food business alone - and created momentum for the take-up of ‘The Co-operative’ fascia and the co-op’s famous dividend card by other societies.

He masterminded the swoops for Somerfield in March 2009 - a deal he first considered when still head of United Co-op - and Britannia Building Society.

If Somerfield gave Marks the springboard to command a food store in every UK postcode, it also put the brakes on like-for-like growth. Food like-for-likes slumped by 2.5% in 2010 and by 2.1% in 2011. And Kantar data shows food sales are continuing to struggle - down 3.6% in the 12 weeks to 8 July 2012, down 2.8% in the 12 weeks to 10 June 2012 and down 3.5% in the 12 weeks to 13 May 2012.

Despite that, Marks will leave behind a much stronger competitor. In a period of economic turmoil and distrust for the banking sector, he’s proved there is a genuine alternative to the major corporates.