Robert McWhir was accused of telling “a complete pack of lies” during his defence testimony at Cambridge Crown Court on Tuesday afternoon.
McWhir, 26, of Potton Road, St Neots, denies murdering mother-of-two Marissa Aldrich on December 22 last year after she drowned in a balancing pond on scrubland at Love's Farm.
During his evidence, McWhir mostly gave one-word responses.
Peter Gair, QC, for the prosecution, told McWhir he had been lying to the court and "knew full well" Ms Aldrich was dead.
"Were your hands on each side of her head while she was struggling to break free so she would drown and die?"
McWhir replied: "No".
On Monday, McWhir told the jury he and Ms Aldrich had been arguing as they walked through the Love's Farm estate and this argument became physical.
He described his relationship with Ms Aldrich as "volatile" and said she was "feisty".
According to McWhir, they wrestled to the floor and rolled down the bank and into the water, which the prosecution described earlier as 10cm deep.
"We were both holding on to each other and I fell into the water," he said.
McWhir, who told the court he had a phobia of water, added: "I tried to feel for the bottom. I let go of her and was flapping my arms around. I thought I was going to drown. Eventually, I found Marissa to my right and tried to clamber over her. Then I felt Luke [Holder] grab hold of me."
However, Mr Gair put to McWhir that he put his "full weight" on Marissa and was "holding her down deliberately".
Mr Gair said: "You had your full weight on her and you were kneeling on her like Luke said, holding her down deliberately so you could drown her."
McWhir replied: "No".
Mr Gair said: "You knew full well she was dead, and dead in your hands. In your eyes she deserved it that's why you made off.
"You then hid from police and when you were spotted by police you jumped on your bike and tried to get off. You knew the game was up so you decided to lie and continued to lie because you had no decent answer to give. You then make up your story that you hoped would fit your account. You have come to this court and told a complete pack of lies to this jury."
The trial continues