An undercover officer who discussed the alleged rape, kidnap and murder of Holly Willoughby with a security guard online engaged with him because he felt there was an “imminent threat” to the TV star, a court has heard.

The officer, who has been granted anonymity to protect his and his family’s safety and “his effectiveness as an undercover officer”, said he had to find out who Ms Willoughby was while speaking with Gavin Plumb.

He told the court the kidnap, rape and murder plot would have been “carried out with or without my services”.

The witness recalled being sent an image of what he described as an “abduction kit”, telling Chelmsford Crown Court: “At that point in the conversation it was quite alarming.”

After being asked whether the fact the defendant is “significantly overweight” would have affected the credibility of his plan to invade Ms Willoughby’s home before “jumping over a wall” when leaving, he replied: “No, it did not.”

The officer, from the Owatonna Police Department in the US state of Minnesota, said he showed Plumb a flight confirmation in their online conversations in order “to show him that I was willing to accomplice him in this act”.

The court was told that, after Plumb was arrested, he refused to provide his PIN (personal identification number), saying: “You don’t need that now.”

He also said to officers: “I mean, based on what you’ve said I can pretty much guess what you’re looking for.”

The court heard that the defendant, described by his barrister as “in the region of 30 stone” when he was taken into custody, later added: “As I said, it’s totally taken me by surprise. I know what it’s about, I can tell you, I reckon, I, I know what it’s about.”

The jury was told the 37-year-old defendant had posted on a messaging application online to say he had a “load of info” on the former This Morning presenter and when she “does and don’t (sic) have security”.

Giving evidence on Thursday, the witness, who used the alias David Nelson as he appeared via video-link from the US, said he messaged Plumb directly when he saw the public post in a chat called “Abduct Lovers”.

He agreed that he assumes “various fake identities” as part of his undercover work and that the one used in this investigation had been “active for about three years” at the time.

During their private discussions, the officer received a selfie image from Plumb as well as a live video in which he said the code word “Holly” to prove he was “real”.

Defence barrister Sasha Wass KC asked: “Would you agree from the picture, you have a man who is significantly overweight?”

The officer replied: “I would say that is rather subjective.”

Ms Wass continued: “Really?”

The witness said: “I can see that he’s significantly overweight by whose terms?”

After the question was put to him again, the undercover officer responded: “I would agree that this is an overweight male.”

Ms Wass then said: “What this person, who you have seen by this stage, is saying to you is that he, together with you, would jump the outer wall of the property.”

The witness replied: “Yes, he is saying he will jump the outer wall.”

The defence barrister went on: “Knowing what he looked like, and the state of his dimensions, did that plan, or what was said about the plan – jumping the outer walls – did that cause you to re-evaluate the credibility of this plan?”

The officer said: “No, it did not.”

Questioned by Ms Wass whether Plumb’s inability to drive and his weight, among other issues, caused the US officer to still believe the alleged plot was credible, the witness said: “Yes, ma’am.

“When you take in the totality… I believe it was a credible plan and I believe this would have been carried out with or without my services.”

The witness told jurors he looks for instances of human trafficking and kidnapping among other things in his role.

The officer said that, through his role, the group “Abduct Lovers” had come to his attention because of its name.

After seeing the public post from Plumb which said he had a “load of info” on Ms Willoughby, he said it had caused him enough “concern” to message him privately.

Under questioning by Ms Wass, the officer agreed that he feared Plumb “must be approaching” Ms Willoughby after reading the post.

The officer said in evidence that he passed information about Plumb to the FBI.

He told the court he asked the defendant how far it was from his address to where Ms Willoughby lived “to figure out where Mr Plumb was located”.

The jury heard that Plumb sent the officer a screenshot of a Google map showing the route from his address to Ms Willoughby’s.

Prosecutor Alison Morgan KC, said that, on October 4 last year, there was a “meeting” with “representatives from the FBI”.

The officer replied “Yes, there was”, and also agreed that it involved contact with the Metropolitan Police.

Ms Morgan said: “And during the course of that liaison between the FBI and the Metropolitan Police, were you providing up-to-date information from the chat you had had with the defendant?”

The officer replied: “Yes.”

He also agreed that the information was “being passed on effectively for authorities in the UK to deal with”.

The undercover officer agreed that the defendant shared “what appeared to be the address” of Ms Willoughby with him online.

Questioned by Ms Morgan, he agreed that Plumb shared “information about the vehicle used by Ms Willoughby”.

He said Plumb also sent a photo of bottles of chloroform.

The officer explained: “Earlier in the conversation he had indicated he had chloroform.

“It was not present in that photo of his abduction kit – I asked where the chloroform was and he sent a picture of chloroform.”

Plumb is accused of attempting to live his “ultimate fantasy” and has been described by the prosecution as someone who had an “obsession” with Ms Willoughby.

The defendant, of Harlow, Essex, denies soliciting murder, incitement to rape and incitement to kidnap.