Della Callagher- a mother-of-one, died two days after eating a 4 course meal at The Railway Hotel near her East London home.
The 46-year-old fell ill as a result of eating turkey that was 'not cooked or reheated properly', an inquest by Havering council heard.
The court took 15 hours to reach a unanimous verdict that the restaurant had failed to comply with requirements.
Speaking about the decision, Havering's councilor Osman Dervish said:
"This is the right result for all of those involved.
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"This was clearly a very serious food poisoning incident which tragically resulted in the death of one of the diners.
"We have put a lot of time into investigating this offence, which has shown that the company did not do everything they could to prevent this outbreak from happening."
A Mitchells & Butlers spokeswoman said: ‘We respect the court’s decision.
"As the jury is still considering charges brought against other defendants in this case, it would be inappropriate for us to comment further."
Verdicts for the former head chef and manager are yet to be decided, and the pub company will be sentenced next month.