An undercover agent has told a court how he exchanged messages with a man accused of plotting to murder Holly Willoughby on a group called Abduct Lovers and believed he was an “imminent threat”.

The US police officer began giving evidence on Thursday in the trial of 37-year-old Essex man Gavin Plumb.

Using the alias David Nelson and appearing anonymously via video link, he said his job was to infiltrate social media to look for suspected kidnapping and human trafficking.

He told…