Image copyright Press Eye Image caption PSNI Chief Constable George Hamilton (left), Deputy Chief Constable Drew Harris (centre) and Assistant Chief Constable Mark Hamilton deny the allegations

Three of Northern Ireland's most senior police officers are under investigation for alleged misconduct in public office and criminality that could amount to conspiracy to pervert the course of justice.

Chief Constable George Hamilton and his deputy Drew Harris are being investigated by the Police Ombudsman.

Assistant Chief Constable Mark Hamilton is also under investigation.

In a statement, the PSNI said they "completely refute the allegations".

Mr Hamilton said he was "absolutely confident" there would be no findings of misconduct against him or the other officers.

The inquiry focuses on concerns about how the Police Service of Northern Ireland conducted an investigation into allegations of bribery and fraud in 2014.

It includes allegations that entries in police notebooks and journals were changed.

Media playback is unsupported on your device Media caption PSNI Chief Constable George Hamilton said he was confident no misconduct would be found

In a statement to the BBC, the ombudsman's office confirmed "a number of allegations" had been made against a range of officers.

BBC News NI has established that those under investigation include:

PSNI Chief Constable George Hamilton

Deputy Chief Constable Drew Harris

Assistant Chief Constable Mark Hamilton

The nature of the complaints and the seniority of those under scrutiny make this investigation unprecedented.

In terms of current policing issues, it's considered to be the most serious investigation the Ombudsman's office has undertaken.

The investigation was launched after the Police Ombudsman, Dr Michael Maguire, received complaints from seven people questioned as part of an investigation into allegations of bribery and misconduct in public office in the awarding of PSNI vehicle contracts.

They included retired PSNI Assistant Chief Constable Duncan McCausland, and the former Chief Constable of West Yorkshire Police, Mark Gilmore.

Image copyright Press Eye Image caption George Hamilton was appointed PSNI Chief Constable in May 2014 - he was previously Assistant Chief Constable

They were questioned in June 2014. Eighteen months later, the public prosecution service informed them that none would face any charges.

The Police Ombudsman has established a dedicated team of six investigators to examine the allegations about the PSNI investigation.

"They include allegations of criminality and misconduct in how this investigation was undertaken," added the Ombudsman's statement.

It's understood the alleged criminality being investigated includes claims that entries in police notebooks and journals were changed.

There are also claims that the PSNI didn't follow proper procedures to obtain warrants.

Image copyright Press Eye Image caption Drew Harris was appointed Deputy Chief Constable in October 2014

A solicitor for those who lodged complaints said he believed there were a number of serious flaws in the way the PSNI conducted the investigation against his clients.

"It is our contention that there is evidence of serious criminal activity on the part of members of the PSNI," said Ernie Waterworth.

"It's an extremely serious allegation and I have to say my clients thought long and hard before going down this road."

Mr Hamilton told the media that he was confident the Police Ombudsman would "vindicate" him and the other officers.

"I've got confidence in the Police Ombudsman, let them do the job, let them vindicate us rather than me saying this because actually that's the way it's supposed to work," he added.

The PSNI normally does not comment in detail on live investigations by the ombudsman, but on this occasion has robustly rejected the allegations.

"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," said its statement.

Image copyright Press Eye Image caption Mark Hamilton was appointed Assistant Chief Constable in July 2013

It said the PSNI "acknowledges and supports the need for office of the Police Ombudsman to investigate these allegations and all officers are co-operating fully with the investigation".

Explaining its unusual decision to give a more detailed response, the statement said media coverage of the investigation "has the potential to negatively impact on public confidence in policing".

Sources have told BBC News NI that the PSNI consulted a number of external criminal justice agencies throughout the 2014 investigation, which it was fully satisfied was conducted properly.

The ombudsman has declared the investigation a "critical incident".

That means it's considered a matter that "could have a significant impact on the person making the complaint, on the police or on the wider community".

The PSNI said it had "full confidence" in the ombudsman to complete a thorough investigation, adding that he should be allowed to do so "without ongoing public commentary".

The investigation is expected to take more than a year to complete.