Justin Timberlake and Jessica Biel Engaged
01/05/2012 AT 02:30 PM EST
Biel and Justin Timberlake are engaged to be married, a source tells PEOPLE.
The big moment happened while the on-again couple were in Big Sky Country, on their annual vacation to Montana, according to the source, who reveals Justin proposed with a "custom-designed" ring.
Timberlake, who turns 31 later this month, and Biel, 29, celebrated the happy news with friends.
Only a few weeks ago, unconfirmed reports said the longtime romantic couple were altar-bound, but the rumors proved premature.
Last year, the couple spent time apart for several months after being together for four years, since May. They made their split official in March 2011.
Still, over that summer, Timberlake told Vanity Fair about Biel, saying, "She is the single-handedly most significant person in my life. In my 30 years, she is the most special person, okay? ... I don't want to say much more, because I have to protect things that are dear to me – for instance, her."
By the following September, the two were back on and publicly affectionate together again.
When they were spotted having brunch last fall in a Colorado mountaintop bar, an onlooker told PEOPLE, "There was plenty of P.D.A., and they seemed happy." Other sightings soon followed in Las Vegas, in New York and in Hollywood.