Busted With Fake Diplomatic Passport

Boris Becker -- the broke ass former tennis star -- claims to have a diplomatic passport from the Central African Republic that gives him immunity in his bankruptcy case in the UK.

There's just one problem -- IT'S FAKE ... at least, according to the C.A.R.

Becker has been flaunting the document ... insisting it's essentially a magic document that protects him from having to pay back the "historic debt" he racked up in Britain.

As we previously reported, a UK court declared Becker bankrupt in 2017 -- and his reps begged the court for mercy claiming he's simply not a "sophisticated man."

Which brings us to the fake passport ...

Officials say it's not just the serial numbers that gave it away -- it…