Britney Spears -- She's Getting Engaged Tonight to Jason Trawick!

Britney Spears is getting engaged to boyfriend Jason Trawick, TMZ has learned.

Multiple sources who are definitely in the know say Jason plans to pop the question tonight ... in Las Vegas.

We're told Britney knows they're getting engaged, so it's not going to be a big surprise.

Sources say Britney's conservators have given their stamp of approval. The matter will go before the judge in the conservatorship case but it's virtually certain Brit will get the judge's sign off.

And we're told ... Jason did it the old fashioned way ... by asking Britney's dad, Jamie Spears, for permission to marry his daughter.

And they're also doing it the new-fashioned way ... we've learned there WILL be a prenup.