Mo Farah's stepdown for a speed tuning 1500m at Sunday's Diamond League meet in Birmingham has been overshadowed by allegations that his renowned coach encouraged doping.

What should have been a celebration of three British athletes who made it 'super Saturday' at the London Olympics -- Farah, long jumper Greg Rutherford and heptathlete Jessica Ennis-Hill -- has been overshadowed by the hard-hitting allegations against Alberto Salazar.

In a documentary screened by the BBC on Wednesday, Salazar is alleged to have encouraged athletes in his care to take banned substances, notably Olympic silver medallist Galen Rupp, the USA's national 10,000 metres record-holder, in 2002.

Salazar also coaches double Olympic, world and European champion Farah in his Nike running camp in Portland, Oregon.

Both Salazar, an athletics great who won three successive New York marathons, and Rupp deny any wrongdoing, while there is no suggestion that Farah has broken doping rules. Indeed, none of the Nike Oregon Project's athletes has ever failed a drug test.

Farah, who set the season's fastest time over 10,000m in Eugene last week, told the BBC: "I have not taken any banned substances and Alberto has never suggested that I take a banned substance."

The 32-year-old Farah has worked with Salazar since 2011, while Rupp, who took silver behind his training partner in the 10,000 metres at the 2012 Olympics, has been a member of Salazar's stable for 14 years.

In Birmingham, Farah will step down to run his first 1500m since setting a European record of 3:28.81 in Monaco in 2013, with Kenya-born American veteran Bernard Lagat also in the field.

"The world championships in Beijing are the main aim for this year. Birmingham will be good to work on my speed in a shorter early season race," said Farah.

"It will be good to step down to 1500m and test myself over a shorter distance, some of the best middle distance runners in the world will be in the field so I will have to be at my best to compete with them."

- World-class field -

Elsewhere on the track at the Alexander Stadium in north Birmingham, there is a world-class line-up in the women's 200m, including the in-form three-time world champion in the discipline, American Allyson Felix, and new British 100m record holder Dina Asher-Smith.

"I'm really happy with my form at the moment but I need to be at my very best again because the other girls in this race are all running well and will be ready if I slip up," said Felix, who scorched to a season-leading 21.98sec in Doha last month.

Ennis-Hill had been planning to contest both the 100m hurdles and shot put, the 29-year-old having recently returned to competition following nearly two years out with injury and then the birth of her first child.

But she was a late withdrawal, arguing the competition came too close to last week's heptathlon in Gotzis.

"Having competed in my first full heptathlon in three years, both (coach) Toni (Minichello) and I feel that my body held up well in Gotzis and we are delighted with how I did, but I feel competing seven days later is going to be too soon," she said.

The 100m hurdles still includes current world champion Brianna Rollins of the United States and Britain's reigning European champion Tiffany Porter, but there will also be no Olympic champion and 2011 world champion Sally Pearson, the Australian having broken her arm in action in Rome on Thursday.

Another absentee, in the men's 800m, is Kenyan world record holder and Olympic champion David Rudisha, who has opted not to risk racing after suffering a leg injury in Ostrava.

The promising cream of British sprinting talent will also be on show, world and European indoor 60m champion Richard Kilty facing European 100m champion Adam Gemili and sub-10sec Chijindu Ujah.

"There are such a great crop of young British sprinters at the moment and it's great that we will have the platform to compete against each other in front of a British crowd," said Kilty.

Jamaicans Nesta Carter and Julian Forte, Frenchman Jimmy Vicaut, American Mike Rodgers and the evergreen Kim Collins of St Kitts and Nevis will ensure a highly competitive race.
Source: AFP