Ponies which have been illegal left on land in Thetford have cost the town council more than £7,000 in just three months.
The authority fenced-off Barnham Cross Common to introduce a programme of grazing animals as part of a 10-year conservation project to promote native plant species.
However since the fencing has been completed, 12 ponies have been illegally left on the common, costing the council £7,620.
The council said the ponies will be removed as soon as possible, to a charity or horse sanctuary, to deter anyone considering illegal grazing.
A spokesman for Thetford Town Council said: 'The council does not like spending money which does not directly benefit council tax payers. However, in this instance we have very little choice.'
The council have said ponies cannot be kept on the common, a Site of Special Scientific Interest and home to more than 60 bird species, as the authority would have to take on legal ownership and associated costs.
The authority had also researched having 'wild' ponies, like Exmoor, on the common but horse welfare charities have estimated the cost of keeping a horses would be around £6,000, which would 'not be suitable' for the council.
Councillors were also concerned that if the horses were allowed to stay, the common could get a 'reputation' and the problems could escalate with more horses being left to illegally graze.
The spokesman added: 'The town council has contracted the grazing out to a local farmer who has the responsibility for the sheep and the cattle.
'At the moment the cattle and sheep are on site all the time.
'However once the majority of the vegetation is removed the animals will only be on the common at certain times of the year, and will have to be removed during the flowering season.
'This would mean that the council would have to pay to have the horses moved and pay for pasture on an alternative site.'
The council are now exploring options of how to prevent horses accessing the common.
This includes height restrictions on the car parks to stop horse boxes from entering.
To report someone leaving a horse on the common, call the council during office hours on 01842 754247 or out of office hours ring the police on 101.
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