The 39-year-old made the decision following his failure to make the cut US PGA Championship overt the weekend.
"Lee needs to try something different with his chipping and putting," said Westwood's manager Chubby Chandler. "He's never been the greatest chipper, but his putting also needs work. He knows that. Parting company with Pete is because he's passing on advice to Lee but he's not around to follow it through.
"Lee is very structured about going to the gym, but not about practising so when Graeme McDowell and Pete's other players are booking him, Lee is leaving it late and is having to work around them.
"Pete was at Kiawah for two days with Graeme this week when Lee wanted to work with him, so it just wasn't working out. Lee's the sort who needs someone standing over him, making him hit chip after chip and telling him what he's doing wrong. It's not like he's got the yips or anything – he just needs to improve his technique.
"When you miss the cut after that, it gets even more frustrating, and that probably brought matters to a head. Lee admits he needed to shake things up because he'd lost his focus and his enthusiasm a bit, because he wasn't getting the rewards for his long game."
Westwood has also opted to split with caddie Michael Waite.