Defending champion and 800m world record holder David Rudisha has pulled out of next month's World Athletics Championships with a quad muscle strain.

The 28-year-old Kenyan said on Twitter it was "sad and disappointing" to miss out on the event, which runs from 4 to 13 August in London.

He is the only person to ever run under one minute 41 seconds for the event.

He won gold in London at the 2012 Olympics and again in Rio last year.

Rudisha said earlier this week he was "excited" to return to London, where he set the 800m world record of one minute 40.91 seconds five years ago.

He is ranked only 19th in the world this year, but would have been bidding for a third successive gold medal at the World Athletics Championships.

Fellow Kenyan Emmanuel Korir and Botswana's Nijel Amos are expected to battle for the gold in Rudisha's absence.