Gary Allen, 47, is on trial at Sheffield Crown Court for the murder of sex workers Samantha Class, 29, in Hull in October 1997 and Alena Grlakova, 38, in Rotherham on Boxing Day 2018.
Allen, of no fixed address, is accused of stamping on Ms Class, before strangling her with a ligature, running her over with a car and then dumping her body in the River Humber on October 26, 1997. Her body was found by schoolchildren on the foreshore of the Humber estuary.
Allen stood trial for Ms Class's murder in January 2000, but was acquitted of the offence.
On Tuesday, the jury at Sheffield Crown Court were played recordings of Allen's conversations with undercover police officers, as part of Operation Misty, which the force launched in 2010 upon Allen returning to live in the area, and also heard from the officers involved.
One of the undercover officer's was known as Ian and had developed a close relationship with Allen over time, telling him about his fictitious involvement in criminality in the Netherlands.
Giving evidence at court, the officer spoke of a conversation with Allen on December 6, 2010. He said: "There was a conversation in a pub where the defendant had been planning to do some work for me. He was planning to kill one of my associates abroad and during that course of the evening he had said to me 'you don't need to worry about me, I have done it before'.
"I stopped him and said I didn't want him to tel me any more as he had had a drink. He brought it up a couple of times.
Allen said to Ian: "I had a bit of string in the back of my car and I said to her 'hang on, I'll just get my wallet and I strangled her'.
Allen told police he had sex with Ms Class after he had been out drinking with a friend in Hull, but said she had got out of his car angry after a condom they had used split.
He denies both counts of murder.
The trial continues.