A diner at Shajan on Longsight Road in Lancashire's Ribble Valley claimed the rodent - later found to be a water shrew - was in their 17 September meal.
Shajan denied wrongdoing and cooperated with the council's environmental health team, which found it "unlikely the rodent had been in the ingredients or cooked due to [its] size".
Damages are sought from eight diners.
It is not known how the water shrew - with a trauma injury on its back possibly caused by a mouse trap - came to be in the curry.
Examination of the shrew's mouth cavity and stomach contents did not reveal any colouration from the spices used in the cooking of curries, the investigation added.
It concluded: "It was obvious [the shrew] would have been spotted during the preparation, cooking, or service of the food."
Potentially defamatory posts were made on social media after the incident, which have since been removed.
A solicitor representing the restaurant confirmed civil action for damages was being taken against eight diners who were part of the group in which the claim was made.
Michael Corrigan added: "We're also contacting police asking them if they could consider whether a criminal offence has been in effect here."
The Indian restaurant's owner Mohammed Ali said he was "relieved" to see "the truth will always come out".
Ribble Valley Borough Council said, like all food outlets, the Shajan Restaurant receives regular environmental health inspections and has had a maximum five-star rating since the introduction of the National Food Hygiene Rating Scheme in 2012.
An Indian restaurant is taking legal action after a claim that a mouse had been found in a vegetable curry.