London, July 23 (AFP) Usain Bolt returns to action after a six-week injury-enforced break in the 100 metres on the opening night of the London Diamond League meeting on Friday with question marks hanging over both his fitness and form just four weeks out from the World Championships in Beijing.

Like Mo Farah and Jessica Ennis-Hill, the British stars of the two-day Anniversary Games, Bolt will be hoping to find the Midas touch that took him to Olympic gold on the same east London track in 2012.

The 28-year-old Jamaican last raced on June 13, when he struggled to get the better of his 19-year-old training partner, the Anguillan-born British recruit Zharnel Hughes, over 200m in the New York Diamond League meeting.

Bolt has since had treatment for a pelvic problem from Dr Hans-Wilhelm Muller-Wohfahrt -- the 72-year-old German sports doctor known as 'Healing Hans', whose diverse list of clients over the years has included Michael Jordan, Diego Maradona, Luciano Pavarotti and Bono -- and has resumed training under long-time coach Glen Mills.

With American Justin Gatlin a clear leader of the world rankings at both 100m (9.74 seconds) and 200m (19.57sec), Bolt desperately needs to rediscover a measure of the form that took him to his second set of Olympic 100m, 200m and 4 x 100m relay golds three years ago.

He has only run one 100m race this year and stands joint 60th in the world rankings courtesy of the modest 10.12 sec that he clocked in winning a challenge event on a specially constructed track at the Rio de Janeiro Jockey Club on April 19.

Bolt, the 100m and 200m world record holder and reigning world champion, has been reported in the Jamaican press to have been "going well" at his European training base at Brunel University in west London but he faces not one but two tests on his comeback, with heats on the schedule at the Olympic Stadium.