The French international tested positive for a substance believed to be a fat-burner after the last-16, second leg clash at Old Trafford, which ended in a 1-1 draw.

The 26-year-old, who joined Liverpool from French club  Paris Saint Germain for a fee of £18 million ($26 million/€23 million) in September 2013, has not been suspended by the Premier League team or by UEFA but his club announced today that he would not feature until the issue was resolved.

Sakho is likely to request the analysis of his B-sample, with the deadline for the player to confirm his intention to do so coming on Tuesday (April 26).

The player could face a lengthy suspension and may miss out on representing his country on home soil at this year's European Championships.

He sat out Liverpool’s 2-2 draw with Newcastle United at Anfield today following discussions between manager Jurgen Klopp and Fenway Sports Group, the club’s owners.

“UEFA would like to confirm the information communicated by Liverpool FC regarding an adverse finding in a doping test of their player Mamadou Sakho conducted at the UEFA Europa League match between Manchester United FC and Liverpool FC on 17 March 2016 (1-1),” a statement from UEFA read.

"The player and the club have received all the pertinent information and have until Tuesday to request the analysis of the B sample as well as to provide explanations for the presence of a prohibited substance in the player's body.

"There are no disciplinary proceedings opened at this stage."

Sakho, who has made 34 appearances for Liverpool this season, scored what proved to be the vital goal in their Europa League quarter-final win over Borussia Dortmund.

The powerful centre-half netted a late header to draw the second leg level at 3-3 before they went on to earn victory against the German team, handing them a 5-4 aggregate triumph.

There is no suggestion Liverpool’s place in the competition is in jeopardy as two players must fail drugs tests a team's result is affected. 

“Yesterday, Friday April 22 2016, a formal communication was received from UEFA stating that they are investigating a possible anti-doping rule violation by Mamadou Sakho,” a Liverpool statement read.

"The player will respond to UEFA on the matter.

"The player is currently not subject to any playing suspension.

“However, the club, in consultation with the player, has decided that while this process is followed the player will not be available for selection for matches.”

Liverpool team-mate Kolo Toure was banned for six months in 2011 while at Manchester City after he had been found to have taken prohibited water tablets which he claimed he got from his wife to help him slim down.