Amantle Montsho of Botswana, the 2011 world champion at 400m, has tested positive for a banned substance at the Commonwealth Games in Glasgow.

The 31-year-old, who won Commonwealth gold in her signature event four years ago, has been provisionally suspended after her A sample, taken after the 400m final on July 29, tested positive for stimulant methylhexaneamine, according to a statement by the Commonwealth Games Federation.

The statement went on to say that Montsho’s B sample will be tested in London on Monday.

Montsho, whose gold medals in Delhi and Daegu were the first in her country’s history, did not medal in the 400m in Glasgow as she tied up badly in the home stretch and finished behind Jamaicans Stephanie McPherson, Novlene Williams-Mills and Christine Day.

The 31-year-old, who has competed in three Olympics without winning a medal, has struggled as this season has progressed. After opening with a then-world-leading 50.87 in April, she has just two podium finishes in five Diamond League races. Her fastest time of the season, 50.37 in Shanghai, currently ranks sixth in the world.

How long of a suspension Montsho could be facing remains unclear.

In 2009, Olympic 100m and 200m silver medalist Yohan Blake of Jamaica tested positive for methylhexaneamine and received a three-month ban. In 2011, Olympic discus champion Sandra Perkovic of Croatia received a six-month suspension for a similar positive test.