Northern Ireland's police chief is facing an investigation into alleged misconduct in public office.

George Hamilton and a number of other senior Police Service of Northern Ireland officers are subject to the probe by the region's Police Ombudsman.

The PSNI said it "completely refutes" all allegations.

The claims relate to how the PSNI handled an inquiry into the awarding of a contract to supply vehicles to the force.

Former West Yorkshire chief constable Mark Gilmore, a former PSNI officer, and retired PSNI assistant chief constable Duncan McCausland were among nine people interviewed by detectives in the 2014 investigation into bribery and misconduct in public office.

No charges were ultimately brought against any of the men, all of whom denied any wrongdoing.

PSNI Chief Constable George Hamilton

Mr Hamilton, current Deputy Chief Constable Drew Harris and current Assistant Chief Constable Mark Hamilton are all now being investigated by Ombudsman Dr Michael Maguire.

A number of other less senior officers are also under investigation.

Dr Maguire has received complaints from a number of those investigated in the vehicle contracts probe in 2014, including Mr McCausland and Mr Gilmore.

It is understood the claims include allegations that police documents were altered.

The Police Ombudsman's Office has begun its inquiry, which could take up to two years to complete.  

The PSNI confirmed that three of the organisation's most senior officers were under investigation and issued a lengthy statement denying any wrongdoing.

Speaking to the media, Mr Hamilton said he had "every confidence in the Police Ombudsman to do a thorough investigation", adding that he believes that he and the other officers involved will be "entirely vindicated".

Mr Hamilton also said that there has been no suggestion of any need for him or the other officers under investigation stepping aside for the duration of the probe.

A spokeswoman said: "The Chief Constable, Deputy Chief Constable and other officers completely refute the allegations made against them and are strongly of the view that these complex investigations into the complainants were conducted with professionalism and integrity. This position has been fully outlined in the officers' initial response to Oponi (Office of Police Ombudsman for Northern Ireland).

"We have full confidence in the Oponi to complete a thorough investigation and we trust that they are left to do so without ongoing public commentary."

Investigation being treated as 'critical incident'

Northern Ireland Police Ombudsman Dr Michael Maguire

The Ombudsman's Office said its investigation would be treated as a "critical incident".

"The Police Ombudsman's Office has begun an investigation into concerns about the way in which the PSNI conducted an investigation into allegations of bribery and misconduct in public office in 2014," a spokesperson said.

"The Office is investigating allegations of criminality and misconduct in how this investigation was undertaken by police.

"It has not named the police officers under investigation, but has confirmed that they include a range of officers, including those above the rank of Chief Superintendent.

"The Police Ombudsman has declared the matter to be a 'critical incident' - an issue the outcome of which could have a significant impact on the person making the complaint, on the police or on the wider community.

"A team has been set up to look into these matters. It includes six investigators and has access to external legal advice

"Given that some of the officers are above the level of Chief Superintendent, the Policing Board (the PSNI's independent oversight body) has been notified.

"The Office has not recommended the suspension of any of the police officers concerned."