Britney Spears' Dad Wants Kevin Federline's Tax Returns to See if He's a Freeloader

Britney Spears' dad is calling Kevin Federline's bluff that he needs more child support to care for the 2 kids he had with the singer ... we've learned Jamie Spears will demand to see Kevin's 2017 tax return to see if he's working for a living or sponging off her.

We've learned K-Fed's lawyer sent Jamie Spears a letter Tuesday demanding her 2017 tax return. Kevin will be getting the exact same letter in the next day or so.

We're told Jamie Spears ain't budging ... he won't agree to pay Kevin a cent over the $20,000 a month he's been getting until Kevin gives him a full accounting on how the money is being spent. Jamie believes Kevin is misusing some of the $20k by spending it on his 4 other children from different women.

So far K-Fed and his lawyer haven't provided such an accounting, although they have demanded more child support. Jamie's position -- the kids needs haven't changed, so the only issue is whether $20k a month cuts it.

We're told Jamie wants to make sure the kids are getting what they need from both Kevin and Britney, and if a judge ultimately says she should pay more, he'll be down with it. But he wants to control the purse strings to make sure Kevin isn't squandering the cash on matters unrelated to Sean and Jayden.