Six kebabs were found to include pork when it had not been declared as an ingredient. Two of the six were described as Halal - food or drink permitted for Muslims, which must not contain pork.
Mr Theobald said it was "totally unacceptable" that people with certain faiths were unknowingly eating meats that were against their beliefs.
The worst doners inspectors came across contained 1,990 calories before salad and sauces - over 95% of a woman's recommended daily calories, 346% of a woman's saturated fat intake and 277% of an adult's daily salt intake.
Researchers uncovered significant regional variations, with the average kebab in the north-west of England containing 1,101 calories, compared with 1,084 in Scotland, 1,055 in Wales and 1,066 in England's south-east.
Northern Ireland's average of 843 was the lowest in the UK.
Mr Theobald said it was "totally unacceptable" that inaccurate labelling was so widespread.
In addition, there was little difference in weight between kebabs labelled as "small" and "large", he added.
"While some people may think they are making sensible choices by ordering a small kebab, this study showed little difference between small and large kebab weight."
He said with obesity rates rising so rapidly in the UK, portion size was as important as content, he said.
"This study has turned the spotlight on doner kebabs and we hope that manufacturers rise to the challenge and work with councils to provide a healthier product that contains only what it says on the label," Mr Theobald said.
Research by the UK's Food Standards Agency in 2006 found that 18.5% of doner takeaways posed a "significant" threat to public health, and 0.8% posed an "imminent" threat.
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