Robert McWhir confessed his love for “on-off” partner Marissa Aldrich days before he allegedly murdered her, a court was told.
In a statement read out in the court on Thursday, the jury heard that McWhir had "confessed his love for Marissa" on December 19, 2018, and the pair had "decided to give it a go".
Ms Aldrich, 29, died from drowning at about 2.30am on December 22, in an area of scrubland at Love's Farm, in St Neots.
McWhir, 26, of Potton Road, St Neots, denies her murder.
Abigail Abraham said in her witness statement that "Marissa seemed fine and happy that night [December 21]."
On the afternoon of December 21, Ms Aldrich told Ms Abraham she was going to make a "go of it" with McWhir.
Ms Abraham said: "She told me that Bobby [McWhir] had declared his love for her on Wednesday and that she had decided she was going to give it a go."
The jury had previously heard how McWhir was "jealous" because Ms Aldrich had been "seeing another man" and that he wasn't going to be "anyone's second best".
The court then heard from PC Paul Chadwick who was on duty the night Ms Aldrich died.
The court was told that police tried to call McWhir's mobile phone after Ms Aldrich's body was found, but he did not pick up.
Police then visited his home in Potton Road, but no-one was in.
PC Chadwick said: "I had heard there had been a sighting of McWhir earlier in the evening."
The court previously heard how McWhir had been seen wandering around a block of flats in School Lane, where Ms Aldrich lived, and, when police approached him, he fled the scene on a bike and rode towards Station Square.
PC Chadwick, who was sat in an unmarked police car, saw McWhir coming towards Dramsell Rise. McWhir was wearing a black Adidas tracksuit with camouflage panels.
As PC Chadwick climbed out of the car, McWhir shouted "Oh, f**k" and tried to turn around and cycle away. He then fell off his bike and was arrested.
The court heard, when questioned by police, how McWhir asked: "who am I supposed to have murdered then?".
When the officer told him, McWhir replied: "I have never heard of her".
The jury had previously been told McWhir had pleaded guilty to attempting to pervert the course of justice after he was heard attempting to blame someone else for Ms Aldrich's death during a telephone call he made from prison, which had been recorded, He also pleaded guilty to breaching a restraining order, the jury have been told, after he smashed a window at Ms Aldrich's flat.
The case continues.