Bart Dalla Mura quit Bernard Matthews last week over frustrations about the slow pace of change at the company, a close source to the former UK chief executive has claimed.

He left unexpectedly after less than a year in the job, without making his reasons public, but a source close to Dalla Mura claimed a disagreement over Bernard Matthews' strategic direction was behind the decision. "You've got a business that desperately needs change to join the 21st century," he said. "Guys like Bart are fast-paced and decisive and the two just didn't gel. The cultural fit was way off."

Founder and chairman Bernard Matthews still had an iron grip on the business, despite announcements that he was stepping back from the day-to-day running, he insisted. "The chairman is still involved, despite ill health. He built Bernard Matthews from a Norfolk turkey farm and still has executive veto on all decisions, despite the existence of the board."

The company refused to comment on Dalla Mura's departure, except to wish him luck. In the short-term, group chief executive Noel Bartram would take on the role of acting CEO, said a spokesman.

"A new commercial head for the UK business will be appointed but in the meantime there is a strong management team in place and it is business as usual," he said.

Dalla Mura was promoted to CEO after joining Bernard Matthews as commercial director in July 2006, shortly after celebrity chef Jamie Oliver criticised the company's Turkey Twizzlers.

Eight months later Bernard Matthews' processing plant in Holton, Suffolk, was hit by an outbreak of avian influenza, thought to have been caused by meat imported from Hungary.

Two weeks after the disease was discovered, consumer spending on fresh turkey plummeted 45%, and by May, lost sales amounted to £9m. Under Dalla Mura's leadership, Bernard Matthews relaunched its products under the slogan Turkey for Today, and committed to a more upmarket range made with British meat. The Grocer was unable to contact Dalla Mura.