Alvaro Bautista Arce missed out on the opportunity to push Rea for the title when he was taken out of Race 2 by clashing with Toprak Razgatlioglu.
That meant Bautista bagged no points for the race, handing his rival the championship victory.
Great Britain
Razgatlioglu takes championship lead with Race 2 victory at Donington Park
04/07/2021 AT 14:19
It was Rea's fifth title in a row.
Earlier, Razgatlioglu had fought back to win the WSBK Superpole in Magny-Cours, pipping Jonathan Rea to the flag.
Yamaha's Michael van der Mark came in third.
The Turkish Puccetti Racing rider had made it two out of two this weekend as he gained 11 places in the opening two laps to set up another battle with his rival from Kawasaki.
An error from Rea on Turn 5 allowed Razgatlioglu past and handed him victory.
Great Britain
Razgatlioglu storms to sensational Race 1 win at Donington Park
04/07/2021 AT 12:22
Great Britain
Razgatlioglu ‘creeps across the line’ to win Race 1 after running out of fuel
03/07/2021 AT 20:24