Serial killer Stephen Port was identified as a “significant witness” with a sexual assault allegation against him hours after his first victim was found dead, an inquest has heard.

The body of Anthony Walgate, 23, was discovered slumped against a wall outside Port’s flat in Cooke Street, Barking, east London, following an anonymous 999 call at 4.05am on June 19 2014.

Port was identified as the caller later that morning and officers spoke to him as a “significant witness”.

However, the first senior officer to attend the scene said he was unaware of a serious allegation against Port dating back to 2012, which had been noted by the borough commander that day.

Over 16 months, Port went on to kill three more young men with overdoses of the drug GHB before he was stopped, an inquest into their deaths has heard.

Giving evidence on Friday, Inspector Gary Learmonth said he arrived at the scene at 4.30am and stayed for four-and-a-half hours.

A “critical incident” was declared as Mr Walgate had a “fat lip” and a “possible footprint” on his torso, jurors heard.

Mr Learmonth said: “We had a young man who was deceased in a public place. It was not clear at that stage how this had occurred.”

He told jurors he assessed the death as “potentially suspicious”.

At 5.16am, records showed Port had been identified as the anonymous caller and his last known address was 62 Cooke Street.

Andrew O’Connor QC, counsel for the coroner, said: “You had his name and you had his address pretty much where you were standing.”

The witness told jurors that officers were keen to speak to him.

When his phone number went to voicemail, officers repeatedly knocked on the door of his flat, with no response.

An officer eventually spoke to Port and took a statement from him at 7.50am.

The inquest was shown a note by the borough commander, Chief Superintendent Andy Ewing, the same day stating “caller previous sex assault”.

Mr Learmonth said he did not recall being told of the allegation against Port, which was printed off the Police National Computer (PNC) a week later.

Mr O’Connor said: “If someone within the police service had found that information that morning would you expect to be told about it?

Mr Learmonth said: “Yes, there is some overlap but yes, potentially I would be one of those persons that would have been updated.”

The PNC report shown to jurors outlined an allegation that Port had raped a male on New Year’s Eve in 2012 at his flat.

A summary of the allegation stated a man had reported that Port had given him “poppers” and had non-consensual anal sex with him.

Mr O’Connor said: “If it is right, that information was discovered that morning, that was a significant piece of information about the person who had called 999?”

The officer said: “Yes, I agree.”

A crime report also shown to jurors revealed the complainant had gone further and described several different occasions of non-consensual sex in which he was plied with drugs and alcohol.

However, no further action was taken after the complainant said he did not support a prosecution, jurors heard.

Metropolitan Police crime scene manager Cheryl Kynaston told the inquest she had noted a small amber bottle on Mr Walgate’s body and thought his death could have been “drug-induced”.

She told jurors no mobile phone or wallet was found and his underpants were back to front and inside out.

Quizzed about the significance of the fashion-conscious student’s underwear, she said: “I thought it was unusual, not necessarily suspicious or sinister.”

She added a busy residential street was a “strange” location.

Ms Kynaston said she thought it likely Mr Walgate died from a drugs overdose.

Peter Skelton QC, for the Met, asked if she considered he had been murdered.

She replied: “No I did not. I thought it most likely he had been moved after he had died but I had nothing to suggest he had actually been murdered.”

In his 999 call, Port claimed not to know what happened to Mr Walgate but later told police they met for a sexual relationship, jurors heard.

He also admitted he had picked him up, dragged him away and propped him up against the wall, jurors were told.

Port was later convicted of lying to police about the circumstances of Mr Walgate’s death, jurors have heard.

Following a trial at the Old Bailey in 2016, Port was found guilty of all four murders and handed a whole life sentence.

The inquest at Barking Town Hall is examining whether police mistakes cost the lives of some of the victims by failing to stop Port sooner.

The hearing was adjourned until Monday morning.