Justin Timberlake

Justin Timberlake has poured out his feelings to Britney Spears in a heartfelt letter sent to the pop star in rehab, according to reports.

A week after Spears' ex called Promises rehab facility in Malibu, California, to offer his support to the troubled singer, he has now put his thoughts on paper.

A Spears source tells Life & Style magazine that the singer was moved to tears as she read Timberlake's letter, in which he offered to be there for her when she completes her stint in rehab.

The insider says, "She could barely catch her breath to read the words. It was one of the most moving letters she had ever received.

"He poured his heart out. He told her he was sorry they fell in love so young, because if they'd fallen in love even just a few years later, he would've married her--and they'd probably have kids together by now.

"Justin let Britney know that she has his unconditional love and support. He offered to come visit her in rehab."

Timberlake ended his three-year romance with Spears in 2002 after the pop star reportedly cheated on him.

He's not only offering his ex personal support; Timberlake has also agreed to help producer pal Timbaland get Spears' pop career back on track.