Jennifer Aniston's boyfriend has asked Courteney Cox to help him pick out an engagement ring.

Jennifer and Justin Theroux have been dating for several months and are blissfully happy.

There has been speculation the two are preparing to tie the knot and Justin is now said to have enlisted Jennifer's best friend Courteney to assist him.

"He didn't want to mess it up and Courteney was happy to help. Justin realises she knows Jen inside out and will give him the best advice," a source told Look magazine.

When Brad Pitt proposed to Jennifer, he presented her with a diamond ring worth over 300,000. Although she grew to love the band, it's been claimed it wasn't quite to her taste.

Courteney is said to have advised Justin to go down a different path. He's decided on a ring worth 10,000, which he believes is perfect.

"It's an exquisite three-carat Asscher-cut diamond ring with a row of smaller diamonds set in a vintage platinum band," the insider added. "The ring is perfect."

Justin is already thought to have asked Jennifer's father John Aniston for her hand in marriage.

"John loves Justin and he would like nothing better than to walk her down the aisle knowing that she'll be marring him," the source said.