Detective Constable Peter Surgay was a family liaison officer following the murder of Mr Collinge in 2011.
He is awaiting the outcome of a two day misconduct hearing held at Nottighamshire police Headquarters at Arnold.
On Thursday Det Con Surgay confessed he was in an on-and-off sexual relationship with Ashfield based defence lawyer Debra Bell, who was representing a defendant in the murder case.
He told the panel he never thought their relationship could jeopardise the case.
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The solicitor, who regularly appears at Mansfield Magistrates’ Court, then went on to represent Stephen Shreeves - one of the three accused of the murder of Mr Collinge.
Shreeves is currently serving a life sentence after being found guilty of murder.
Widow Charlotte Collinge was initially found guilty of murder but was later acquitted following a retrial last year.
Mr Surgay’s relationship with Debra Bell was kept secret as his wife also served with Nottinghamshire Police, the hearing was told.
The panel heard Mr Surgay and Ms Bell went on holidays to California, Cornwall and Dublin during a five year relationship.
The officer maintained it was a platonic relationship apart from a “drunken fumble” early on.
Mr Surgay is alleged to have not reported the nature of the relationship which, it was argued, could have compromised a major investigation.
The hearing was told DC Surgay, who had been with the force for 20 years had received a commendation for his work on one murder Inquiry and had been recommended for another.
He told the panel that despite his years of experience he did not think his relationship with the defence lawyer was something he would have to report to prevent him being compromised.
They had never spoken about work and he was unaware of the policy to report it.
Force solicitor David Ring said it was a matter of commonsense that the family liaison officer was in a position of trust and must have the trust of the family.
During cross examination Det Con Surgay said if the family had found out he thought they would have supported him.
He said: “I gave 100 per cent to the family and I earned their trust. I don’t believe that would have been lost.”
During cross examination David Ring suggested the officer had downplayed the sexual part of their relationship and the holidays when interviewed as part of the investigation.
He suggested the officer had tried to cover it up so his wife would not find out and he would not be taken off an investigation he was involved in.
Mr Surgay said; “I wasn’t aware that association with a solicitor was a compromise. They are upstanding members of the community.”
He admitted the relationship was not something he wanted to “shout from the rooftops”
Mr Surgay said he had not brought up the sexual element or their holidays during his interview as he had been expecting it to be put to him later.
He said: “I don’t seek to hide away from those facts.”
Det Con Surgay is also facing a separate allegation of gross misconduct for sending “appalling” text messages to Ms Bell after the breakdown of their relationship.
He said he regretted sending rambling abusive texts to her after the defendants convictions were quashed and a retrial was ordered.
The family’s reaction to the news had left an “indelible mark” on him and he was not in a rational state of mind.
The texts had been rambling and incoherent and he had sent her a stream of apologies on the day after.
The officer is also accused of accessing confidential information on police computer systems over an extended period of time without lawful authority or reasonable excuse and otherwise than for policing purpose.
Instances were detailed of him searching for his own surname, crimes around the area he lived in Kimberley.
He told the panel he was unaware of the policies regarding the use of computer systems and said his unauthorised searches were always about detecting crime.
The hearing will reconvene on Wednesday August 10, when the outcome of the case is expected to be decided.