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Scotland Yard’s elite unit at the centre of the Plebgate scandal was today facing fresh controversy as it emerged a black woman officer is suing for alleged racial discrimination.

Carol Howard, 34, a Diplomatic Protection Group (DPG) firearms officer, who helped safeguard London from terror attacks during the Olympics and protects senior politicians, government buildings and foreign embassies in London, claims her career has been held back.

She is alleging a senior male DPG officer has denied her equal opportunities because of her race and gender.

The tribunal hearing, set to take place at Central London Employment Tribunal next month, also involves allegations of bullying.

A source told the Standard the case, which is being funded by the Police Federation, has “been going on for two years”.

Ms Howard, who is based at Kensington, has served in the force for more than ten years and was featured in the Standard in 2012 as one of the Met’s officers who helped keep London safe during the Olympics. She has been taken off frontline duties.

A Met spokesman said: “Police Constable Carol Howard of the Diplomatic Protection Group is bringing an employment tribunal against the Met citing racial and sexual discrimination.

“The Met will robustly defend the claim.”

The issue of racism is particularly sensitive for the Met. Last year, on the eve of the 20th anniversary of Stephen Lawrence’s death, Scotland Yard’s black and Asian officers declared the force is still “institutionally racist”.

The Metropolitan Black Police Association (BPA) said that despite training, more needed to be done to tackle the mindset that led to failures over the Lawrence case.

On Monday, speaking 15 years after the Macpherson Report also claimed the Met was “institutionally racist”, Baroness Lawrence, Stephen’s mother, said she believes some attitudes “haven’t changed much”.

However, Met Commissioner Sir Bernard Hogan-Howe has vowed to become the “implacable enemy” of racists within the force, and responded to BPA’s assertions last year by saying: “I think there is lots of evidence to say it isn’t true and that we’re actually doing a pretty good job and we are improving all the time.”

The racial discrimination hearing is the latest in a series of allegations to rock the DPG after two of its officers were sacked on Wednesday for their role in the Plebgate scandal involving former chief whip Andrew Mitchell at the gates of Downing Street.

PC Keith Wallis, who was sentenced to 12 months in prison on February 6 for lying about witnessing the incident, was dismissed from the Met.

PC James Glanville was also dismissed at Wednesday’s misconduct hearing after being arrested on suspicion of giving the Sun newspaper information about the incident. He has not been charged.

The PCs’ dismissals come after three officers in the unit were arrested over claims they exchanged “extreme” pornography with each other on their mobile phones.

They were arrested on December 19 with a fourth questioned on January 8. They no longer face criminal charges but are still subject to an investigation into allegations of misconduct.

The DPG has more than 700 police officers, many of whom are authorised to carry firearms, who protect the most sensitive government buildings, senior politicians and foreign embassies in London.