The funnyman has fallen ill just days before he is set to reprise his role as host for the third consecutive year at the music ceremony on Wednesday at London's O2 Arena.
A source tells the Daily Mirror newspaper: "James has suffered with it (tonsillitis) in the past so recognised the symptoms straight away. He knew he had to act fast and rest immediately to stop the viral infection developing any further.
"James has been really excited about presenting the BRIT Awards and doesn't want to have to pull out so getting better in time for the big night is his number one priority."
This is not the first time Corden has been plagued by health issues while hosting the BRITs - last year, he sustained an injury and had injections in his knee to help him stand onstage throughout the prizegiving.
In 2011, he suffered a scratched cornea when firework dust from Take That's BRITs performance became lodged in his eye.