Jessica Simpson and MAROON 5 singer Adam Levine first became romantically involved in 2004, according to two eyewitness who have come forward with shocking new revelations.

Although Simpson and husband Nick Lachey battled infidelity rumours throughout their marriage, new information seems to confirm the marriage was in trouble long before the couple separated in November (05).

According to photographer ANDrew SWEENEY and reporter LISA LADABOUCHE, Simpson and Levine got together after they both performed at a concert in Boston, Massachusetts, for radio station Kiss 108 FM in May 2004.

Simpson was attending the concert without her husband, who was on holiday in Fiji with his brother, DREW.

Sweeney and LadaBouche, who were covering the concert for local paper, The Raw Story, tell American publication Us Weekly they witnessed the two singers kissing in a VIP area backstage.

Sweeney says, "They were backstage flirting. He was touching her hand. And then they just leaned in and started going at it, making out.

"Not horribly, like teenage making out, but like adults, little kisses."

LadaBouche agrees with Sweeney saying, "I definitely saw them kissing. It was a shock - especially knowing her husband was away.

"And she's supposed to be this innocent figure."

After the concert, members of Maroon 5 and the Backstreet Boys, who had also performed at the show, went to local Boston nightclub Saint, for an afterparty, where the celebrity couple continued to get cozy.

Saint owner BRIAN LESSER confirms, "Jessica and Adam were sitting next to each other. I think that was the beginning of it all."