The Met Police told staff to delete records on sex and race discrimination against one of its employees, an employment tribunal has found.
Firearms officer Carol Howard, 35, was “singled out and targeted” for nearly a year, a panel ruled.
An officer looking at her complaints was asked to delete references in a report into discrimination related to race or sex, it said.
The Met said it was “disappointed” at the findings but would review the case.
Ms Howard, of Purley, south London, brought a claim of discrimination at the Central London Employment Tribunal earlier this year.
A judgement issued by the panel which heard the case said the Met “directly discriminated” against Ms Howard “on the grounds of sex and race” between 31 January and 29 October 2012.
A number of Ms Howard's complaints of “victimisation” were “well-founded”, the tribunal added.
The force concluded there was no evidence “without having conducted a proper investigation”, it said.
It also found that a detective sergeant tasked with looking at Ms Howard's fairness at work (FAW) complaint was asked to delete references to discrimination and harassment relating to sex or race in a report.
The judgement concluded this was done “not because they were not supported by evidence in the report, but because the claimant had brought a complaint of race and sex discrimination in the tribunal”.
The tribunal recorded that it was “very concerned the [Metropolitan Police's] policy of not allowing Fairness at Work Advisers to make assessments of discrimination and of instructing to delete them when they do so, might mislead complainants and tribunals into believing that the [investigating officer] has not found any discrimination when in fact he or she has done so”.
The 35-year-old worked in the Diplomatic Protection Group (DPG), which provides protection for foreign embassies and missions in London.
Her superior, acting Insp Dave Kelly, subjected her to “a course of conduct which was detrimental to her”, the panel said.
Within weeks of becoming her line manager Insp Kelly “formed the view… that the claimant was dishonest and not up to the standard required for DPG”, but the panel said he had not provided a “credible basis for forming such a view”.
Her commitment was challenged in front of colleagues, her “every absence” was assumed not to be genuine and her application for an armed response vehicle role was not supported, the panel found.
Lawyers for Ms Howard will now seek compensation from the Met Police for injury to feelings and aggravated damages.
Her lawyer Kiran Daurka, from Slater & Gordon, called for a “complete rewrite” of the force's equality procedures.
“The conduct of the Metropolitan Police and some of its senior officers towards Carol Howard was deplorable over the last two years,” she said.
“My client was subjected to discriminatory treatment because she is black and because she is a woman.
“Fifteen years after the Met was branded 'institutionally racist' they have failed in addressing discrimination which pervades the system.”
And she told BBC Radio 4's Today programme that the deletion of records of sex and race discrimination was a policy within the Met.
“We anticipate that wherever there's findings of discrimination, they're being instructed to delete them. They've admitted that they do this practice… so that there are no findings of discrimination against them.”
Daphne Romney QC added: “What is particularly shocking is the deliberate attempt to cover up internal findings of discrimination.
“The outcome of the internal grievance investigation led PC Howard to believe that the investigating officer did not accept that there had been either race or sex discrimination, when clearly this was not the case.”
In a statement, the Met Police said: “We are disappointed at the tribunal's finding in favour of PC Howard.
“We will review the findings, take legal advice and take forward any learning or actions as appropriate.”