The 20-year-old, who held the record with a 69.59 throw, is now within touching distance of qualifying for the 2012 London Olympics.
The Burnley-born athlete needs a second throw of over 69-metres to qualify, a feat she comfortably achieved in 2011.
After her record-breaking throw, Hitchon modestly tweeted: "70.40 good start to the season."
Current world record holder Betty Heidler of Germany set the landmark of 79.42m last year at the age of 27, but she did not hit the distances Hitchon has already achieved until the age of 22.
Hitchon has excelled in the sport after switching from ballet at the age of 14.
Sophie Hitchon has set a new British hammer record with a throw of 70.40m at a meeting in Loughborough.