Britney Spears and boyfriend Jason Trawick are supposed to get engaged... tonight!

TMZ has learned from multiple sources that Trawick plans to pop the question the pop star tonight in Las Vegas. learned that Trawick has already purchased and taken delivery of a diamond bauble for the blonde mother of two who just celebrated her thirtieth birthday.

Don't expect anything too massive or obnoxious, the source explained to exclusively.  This ring will be classy and under 4-carats.

Trawick went to Spears' father, Jamie Spears, to ask for Britney's hand in marriage.

Spears supposedly already knew that they would be getting engaged, so the news comes as no surprise.

Jason definitely has the engagement ring and is ready to propose to Britney, the source told RadarOnline.

She isn't going to be disappointed.

The sources told TMZ that Spears' conservators have already given Trawick the go-ahead. The engagement does have to go before the judge in the conservatorship, but TMZ reports that it is almost certain he or she will approve it.

TMZ has also learned that there will be a prenup.

Spears has been on the last leg of her Femme Fatale tour. Trawick has been accompanying her.