Strathclyde Police has exceeded its budget for legal assistance by £336,000 so far in 2012/13 – pointing to an increase in the number of officers facing criminal allegations.
The force said it was concerned by the unexpected increase, adding that some cases had been "more complex and involved", while politicians have called for an investigation into why there has been such a jump in cost to the taxpayer.
Scottish Conservative Chief Whip John Lamont said: "This rise would suggest a jump in the number of officers facing criminal charges.
"The public will be rightly concerned that not only are more officers being accused of breaking the law but that it is costing so much public money.
"Strathclyde Police has to investigate as a matter of urgency as to why there has been such an increase in these cases, both to restore public confidence and to bring down the cost to the taxpayer."
Lewis Macdonald, Scottish Labour's justice spokesman, said: "We have to first understand why there has been such a significant rise in legal costs for police officers and we would be concerned if this is matched by a rise in police officers being charged with crimes.
"At a time when public money is tight, people will not see this as a good use of funds and we would expect the police to take action to bring down costs."
The figures, which cover the period from April 1 to November 30, were announced at a meeting of the Strathclyde Police Authority, where finance director Allan Macleod said the overspend was a new trend.
Mr Macleod said: "There has been a significant overspend in legal fees for police officers defending criminal accusations."
He added that an extra £136,070 had already been paid out, with a further £200,000 outstanding. However, the overspend will still be covered by the authority's overall budget as other costs have come in below target.
A spokeswoman for the force said: "Strathclyde Police is concerned about the rising costs of legal expenses in respect of criminal proceedings brought against police officers.
"This has reached £336,000 this year and is a significant spend of public funds."
She added: "Where criminal proceedings are brought against a police officer in respect of any act alleged to have been committed in the execution of police duty, the police officers can apply for legal assistance from the police authority for his or her defence."
The decision to award legal fees is the responsibility of the deputy chief constable, who looks at whether the charge is likely to give rise to a civil action against the force.
He also considers whether or not there will be an impact on discipline within the force if assistance is refused and whether the assistance given is reasonable.
The police chief also looks at whether the officer concerned has acted in good faith in the execution of his or her duty.
The force spokeswoman added: "It is important that an officer who has acted in good faith has access to legal funding and Strathclyde Police is content that the decisions taken by the deputy chief constable and the force in relation to the funding of legal expenses are sound, consistent and fair with due regard to the set criteria.
"When funding is refused, it is open to the officer concerned or his or her representative to appeal that decision to the police authority which may uphold that decision or overturn the refusal of the deputy chief constable to grant funding."