Dormice could prevent the move of a football club and the building of a new supermarket in Cornwall.
Morrisons wants to build the store in Wadebridge on the local football club's practice pitches and put up a new ground on farmland outside the town.
A council planning officer said there was "reasonable likelihood" the Bodieve site may be home to the rare species.
Members of the council will meet on Thursday to decide whether the scheme can go ahead.
Wadebridge Town Football Club is keen to expand onto a new site at Bodieve, just to the west of the town, in a deal financed by Morrisons, which will see six new pitches, floodlights and car parking.
The supermarket chain would then develop the existing football pitches opposite the town's sports centre.
The South West Peninsula League club has made the planning application.
But council planning case officer Gavin Smith said dormice had not been completely ruled out as being on the site and had been found about 1.8km (1.1 miles) away.
Dormice normally inhabit woodland with scrub and hedgerows and are protected under European law.
'Head for farms'
Mr Smith said he could not recommended the application was approved, despite the fact it would benefit the community.
In a report to the council, he said: "There is a reasonable likelihood that dormice are present on the application site and the applicant has failed to establish the extent to which the proposal will impact upon this European Protected Species."
The chairman of Wadebridge Town FC, Steve Cudmore, said he thought the dormice might be far enough away not to cause a problem.
He said: "One would say that if the dormouse is 1.8km away, it would head for farms, rather than a little hamlet at Bodieve."
A survey for dormice in the area is being carried out on behalf of the Cornwall Wildlife Trust, but it is not due until November.