“It’s vicious,” says Co-operative Group chief executive Peter Marks of competition in the grocery market which has changed “beyond recognition” in his 45 years with the business.

Marks’ decision to step down next May comes as The Co-operative Food faces as stiff a challenge as it ever has – keeping shoppers as the big four grocers muscle into the convenience market to varying extents.

But his decision makes sense for the group. “I spoke to my chairman last year and we decided that it might affect the Verde deal [to buy 632 Lloyds Banking Group branches] and now that’s complete it feels the right time,” he tells Retail Week. “It will take 3-5 years of 24/7 work to integrate that business and I don’t have that in me.”

Marks is modest about his legacy but believes integration has been the key word which defines his six years as chief executive. “It was a fragmented ragbag of individual organisations almost competing with each other,” he says. “We were losing market share, influence and brand identity. I feel very positive about it now, not everybody agrees with the model but we now have 90% of the co-operatives in the UK and have the scale to invest.”

Planet Retail UK grocery analyst David Gray agrees. “His key legacy is unification,” he says. “Creating one brand from so many businesses trading under different names was a good decision.”

In grocery, Marks will be defined by the acquisition of Somerfield. Originally heralded with fanfare as The Co-op’s true arrival in a new ‘Big Five’, the integration quickly became difficult with bringing operations and staffing together proving a mammoth task.

Marks is bullish about the acquisition. “Yes, it’s been tough,” he says. “We bought the business in 2008 and we have the worst financial crisis that this country has ever seen the same year. Five years on and we’re in a depression, integrating a business has not been easy in these horrendous conditions.”

Gray says: “Somerfield has been very, very challenging. In context Somerfield was an enormous business and integration problems were likely as we saw with Morrisons and Safeway before that.”

It has not always been easy for Marks. He’s cut a defiant figure and has been under pressure at times in recent years. The keen musician says he is looking forward to “having a break, relaxing, playing some golf and drumming”.

The 62-year old is not giving up corporate life though. He intends to satisfy his fervour for business with several non-executive positions to add to his non-executive position with Thomas Cook.

So what lies ahead for Marks’ successor?

A combination of what Marks dubs “the worst economic conditions ever seen” and increased competition have ebbed away at The Co-op’s market share. Worringly for The Co-op this share drop comes despite the fact the convenience market is booming as people shop more frequently but with smaller baskets.

Gray believes an overhaul of its image is needed. “They need to do something about their food stores. Their strapline – Good with Food – is tired as are some of their stores. They need to tighten their game. They need to revitalise the whole business,” he says.

The Co-operative Food’s new chief executive Steve Murrells has plenty on his plate then and analysts believe that the group could make further acquisitions after a £950m refinancing deal was struck last week.

Marks has put what was once the UK’s largest retailer back on the map after a long and difficult period. What lies ahead for the retailer could be a similarly challenging period.