Britain’s most scandal-hit police force faces a slew of legal claims after being accused of using controversial anti-terrorism powers to snoop on officers blowing the whistle on racism.

Cleveland constabulary faces claims that it secretly obtained details from confidential emails between Asian officers and their representatives and solicitors to defend against employment cases brought against the force.

It is the latest scandal involving alleged police misuse of the Regulation of Investigatory Powers Act (Ripa). Last month, Scotland Yard was ruled to have illegally accessed the phone records of a Sun journalist while using Ripa to find the source of a leak during the “Plebgate” scandal. The legal challenges – set to be filed imminently – are yet another setback for Cleveland Police. The then chief constable was sacked in 2012 for gross misconduct, and in November an employment tribunal exposed a racist campaign that targeted an Asian firearms officer.

The ordeal endured by Nadeem Saddique, 44, highlighted racist attitudes that the force’s own disciplinary unit failed to investigate properly, the tribunal found. Fellow officers had described him as “just a Paki” and pledged to oust him from the unit.

PC Saddique had written emails in 2012 to a senior ethnic minority officer after a white colleague sported an English Defence League sticker on the holster of his weapon. The sticker reputedly included the phrase: “Crusades against Muslims, tell me you’re a Muslim and I’ll shoot you.”

The Independent can reveal emails from Mr Saddique were part of a cache of documents that Cleveland Police secretly sought to recover from his senior ethnic minority colleague using the Ripa laws during a criminal investigation into unauthorised leaks from the force. The senior officer has since left the force and is preparing his own legal claim.

The force also used Ripa, which was passed in 2000 to help investigate terrorism and serious crimes, to check phone data of police officers, a solicitor and journalists over a five-month period, according to documents secured by one of the targets of the inquiry.

It is understood that lawyers and former police officers are all preparing claims against the force alleging “misfeasance in public office” over the potential viewing of legally privileged documents.

The trawl in 2012 would have allowed the force to see emails containing details about at least three Asian officers claiming discrimination, according to sources.

Critics say powers have been abused The Regulation of Investigatory Powers Act, made law in 2000, was designed to help the authorities to fight serious crime. It empowered public bodies, including police and local councils, to carry out surveillance and intercept electronic communications. However, critics say the powers have been abused, with officials using the act to spy on suspected littering and other minor offences. Last year, ministers tightened the rules to require police to get a judge’s permission to obtain journalists’ phone records, after Scotland Yard accessed the records of Sun reporters to find who had leaked information on the “plebgate” row between police and former Chief Whip Andrew Mitchell.

The Police Federation – the group which represents rank-and-file officers in England and Wales – has separately launched an official complaint to the police watchdog about the trawl of contacts of Steve Matthews, a senior official in Cleveland. The federation is representing a number of other officers who are bringing forward discrimination claims against the force.

Cleveland Police said the watchdog, the Independent Police Complaints Commission, has passed it details of the complaint “very recently … to assess, which we will do over the coming days”.

Mr Matthews, the former chairman of the Cleveland branch of the Police Federation, confirmed that he was preparing to serve papers on the force. “I’m disappointed that I now discover that while dealing with the force they were snooping on my phone calls and the people I was representing, which is a step too far. As a consequence, I’ve taken legal advice and it will lead to me taking action against the force I served once so proudly.”

Nobody was charged as a result of the phone and email trawling exercise and the secret retrieval of phone data only came to light in October 2015.

“It’s not just excessive but, on the evidence I have seen, to be a clear abuse of power,” said Sultan Alam, a former Cleveland officer who was wrongfully imprisoned by the force after he launched a discrimination claim. “For that length of time it’s just bizarre – it seems as if it was a fishing exercise.”

Mr Saddique – who had protected Tony Blair and members of the Royal Family – took his employer to an employment tribunal last year. The tribunal found 19 instances of victimisation by individuals and another 11 of direct discrimination.

Following the Saddique tribunal hearing, the Cleveland Police and Crime Commissioner (PCC) Barry Coppinger apologised to the officer for his treatment. But at the time Mr Coppinger said there was no suggestion “that operational policing to the public is delivered in a way which is affected by unlawful discrimination”.

However a statement by Mr Saddique alleged officers adopted racist attitudes against members of the public, including:

one officer boasted how he called in a dog team to submit a random Asian driver to a drugs search even though he knew no drugs were in the car;

another officer said he would go down to the “local mosque and shoot the Paki kids”;

a third officer put up pictures on the office wall of an Asian man caught for speeding – the only person ever identified by the officer in such a way;

another two officers made no bones about targeting “fucking Paki drivers” for road offences.

Mr Coppinger told The Independent: “I’m not familiar with all the detail in the case. You’re giving me details from a statement that I haven’t seen.”

Mr Saddique’s original claims about racism were not properly investigated by the force’s professional standards department, despite a request by the outgoing Chief Constable Jacqui Cheer, who had sat down with the officer and his MP, James Wharton, to promise one. “At that meeting I was given assurances that my constituent’s concerns would be properly looked into,” said Mr Wharton, the Conservative MP for Stockton South. “We now know that didn’t happen. That raises very serious questions that go to the very top of the local police organisation.” Jacqui Cheer will leave policing on 12 January after 31 years to teach at the College of Policing’s strategic command course.

Mr Wharton is due to meet Mr Coppinger to demand that inspectors are brought in to investigate problems within the force. “There’s something at the heart of Cleveland Police which may be an institutionalised problem and the PCC needs to get a grip on it because public confidence has been hurt now over many years to the point at which many people feel Cleveland Police is not an organisation they can trust,” he said.

Cleveland Police’s Ripa trawling operation, if proven, threatens to be more controversial than the Metropolitan Police’s use of Ripa to seize journalists’ phone records during the investigation into the source of the “Plebgate” story. In that case, Scotland Yard initially only seized phone contacts covering seven days – and only used Ripa powers to secure the records after evidence emerged of a potential conspiracy to bring down the cabinet minister Andrew Mitchell.

The Investigatory Powers Tribunal ruled last month that it was “neither necessary nor proportionate” for the Met Police to have secured the phone records of one reporter, but found that it was justified in three other cases.