Sussex Police Chief Inspector Rob Leet is said to have met Sergeant Sarah Porter for a romantic encounter on at least one occasion in 2017 while they were working.
He has also been accused of having sex with a victim of crime years earlier.
The Independent Office for Police Conduct (IOPC) announced the findings of its investigation on Monday.
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Leet was suspended from duty while the investigation continued.
The married father-of-four previously said the claims regarding his liaison with Porter were unfounded.
They are accused of travelling to meet each other while on duty when there was no work-related reason to do so and "repeatedly" using police systems to exchange personal messages, a police watchdog spokesman said.
After the IOPC launched the investigation in March 2017 - and following publicity of the case - a victim of domestic violence came forward with the further allegations, the Press Association understands.
This means Leet now also stands accused of being intimate with the victim in 2014 and 2016.
Porter was unable to be contacted when a fatal crash happened while she was on duty and approached a witness in the IOPC investigation "inappropriately", it is also alleged.
The IOPC said: "We concluded that if proven, the behaviour would be a breach of standards of authority, respect and courtesy and duties and responsibilities and could amount to gross misconduct.
"Sussex Police agreed and a hearing will be scheduled in due course."
The force has been embroiled in a string of sex scandals in recent years.
In October former Brighton-based police constable Alexander Walsh was handed a community order for stalking and common assault after unleashing a barrage of sexual propositions on a colleague when she was held in a patrol car with him for more than an hour after dark.
The former Metropolitan Police officer, who then joined the Sussex force, was spotted by fellow officers groping the woman on a work night out before badgering her with suggestive texts and taking pictures of her without her consent.
That same month prosecutors dropped a case against Brighton-based Inspector Tony Lumb, who faced allegations he had sex with women he met while on duty.
The former elected member of the local Police Federation branch's board was arrested on suspicion of misconduct in public office, interviewed under caution, released on bail and suspended from the force pending an IOPC investigation.
The police watchdog said it found evidence to suggest a criminal offence may have been committed.
The CPS has been asked to review its charging decision after one of the complainants, 52-year-old online abuse campaigner Nicola Brookes, called the news "shocking".
Lumb could still face disciplinary proceedings.
In April a second police officer was caught selling himself for sex while on sick leave.
Detective Constable Richard Holder was sacked without notice for gross misconduct at a disciplinary hearing but had already resigned.
This came after Hastings-based police constable Daniel Moss was investigated and suspended from duty in December 2016 after he was caught advertising himself online as a male prostitute and offered to perform sex acts for cash.
He had been on sick leave since that September for stress.
He also failed to attend misconduct proceedings and was dismissed with immediate effect.
In March sexual offences liaison officer and police constable Martin Harris was jailed for two years for misconduct in public office, downloading and making indecent images of children.
He said he found a rape victim in his care "attractive" and had hacked into her Facebook account to download pictures of her as a child.
Police constable Mark Scruby was sacked from the force in 2017 after telling his sergeant she resembled a porn star.
In 2015 Inspector Lee Lyons was fired after admitting he contacted prostitutes while on duty.
The force has been contacted for comment.