The cover price cut took effect this weekend (13 November) in the Granada, Yorkshire and Tyne Tees regions.
It follows a similar price cut in Scotland, which took effect from 23 October.
The Sun is the country’s biggest selling daily title, boasting a daily circulation of 2.9 million in September, according to the latest audited figures.
The Audit Bureau of Circulations (ABCs) does not breakdown Saturday circulation figures, but it is thought that it will cost the company hundreds of thousands of pounds in weekly cover price revenue.
The move by News International has been matched by the Richard Desmond-owned Saturday edition of the Daily Star, which is reducing its cover price from 50p to 30p in the same regions.
The move by News International comes after the September launch of The Sun’s new weekend TV magazine, Buzz, with a star-studded TV advertising campaign.