Chris Fox, the head of the force, appeared to directly contradict the Bassetlaw MP's account of how he was found in a massage parlour during a police raid.
Confirming for the first time that Mr Ashton was on the premises in the Northampton parlour, Mr Fox said that his officers had been forced to run a computer check to check the MP's identity.
Mr Fox said that Mr Ashton was subsequently interviewed by police in the presence of his solicitor and declined to give a witness statement.
The Police Constable said he had decided to give more details about the raid on the Thai massage parlour in Northampton last November because much of the information was already in the public domain.
Four people were arrested on suspicion of committing sexual offences and a number of Thai women were deported for breaches in the immigration rules.
The police announcement came a day after Mr Ashton denied that he had taken part in or paid for any sexual services at the parlour and said he had not given "inaccurate particulars" of his name and address.
The 65-year-old MP said on Monday that he was considering calling for a full Police Complaints Authority inquiry into how his name had been leaked to newspapers.
He claimed that the reports in Sunday tabloid newspapers could have only come from Northamptonshire Police and warned that he might take the matter up with the Data Protection Registrar.
Mr Fox said yesterday that if any member of his staff was involved in leaking information to the media about the raid they would be "dealt with in the strongest possible way".
No official complaint had been received, but he said that the force had its reputation and integrity to protect and would treat seriously any allegations of breaches of confidentiality.
"Allegations of this kind can be extremely damaging to the force and I have yet to find out why people are so sure that any leaks must have come from Northamptonshire police," he said. "There were a great many other people with knowledge of this case - defendants, witnesses, legal representatives, other organisations and other individuals."
He would be writing to Mr Ashton to ask him to substantiate his allegations that his police force might have been responsible for leaking details.
A statement by the force said: "When questioned at the weekend about Mr Ashton's presence at the massage parlour during the raid, Northamptonshire Police confirmed that one man was present when officers executed their warrant.
"They said he was not committing any offence and that he declined to assist the police as a witness, which he was perfectly entitled to do.
"However, now that it is in the public domain, Northamptonshire Police confirm that Mr Ashton was present at the massage parlour when the warrants were executed."
The Labour Party refused to comment on the Chief Constable's statement.