Pacquiao sued by accounting firm

According to TMZ.com Manny Pacquiao’s former accountants claim they’ve discovered the boxer’s BIGGEST WEAKNESS … he sucks at paying his taxes … this according to a new lawsuit obtained by TMZ.

The number crunchers claim they were called in to save PacMan’s ass last year … because, according to the suit, Manny’s tax issues “seriously affected [his] ability to fight.”

The accountants claim they straightened out his money problems — and even let Pacquiao borrow some cash — but Manny never paid the them back and totally ignored the bill.

Now the pencil pushers are suing for their jack — $649,016.95.

Calls to Manny’s people have not been returned.