Scotland Yard is facing court action next week after refusing to hand over the results of investigations it was ordered to conduct into claims that it used counter-terrorism powers to discriminate against and harass innocent Muslims.

The Independent Police Complaints Commission (IPCC) is seeking a high court order against the refusal, which it says amounts to a clear breach of the law.

The IPCC ordered the investigations following complaints from Muslim Britons alleging police discrimination in the use of schedule 7 of the Terrorism Act, 2000, which allows stops at airports and ports, even if wrongdoing is not suspected.

Muslims are disproportionately targeted but the police deny they use ethnic or religious profiling. Scotland Yard is the only police force refusing to tell the IPCC the result of its investigation, the watchdog said. Schedule 7 was also used last month to detain, question and search David Miranda, the partner of the Guardian journalist Glenn Greenwald, who has revealed mass surveillance programmes by the US National Security Agency. The Met said it is considering the IPCC's action.

The IPCC began inquiries last year after receiving complaints from the group CagePrisoners acting for, among others, eight Muslims stopped under schedule 7. They claimed they were subject to discriminatory treatment and were targeted by police unlawfully because of their religion.

The IPCC decided to supervise an investigation by Scotland Yard.

Letters seen by the Guardian show haggling between the watchdog and the Met over the scope of the investigation. The watchdog sent a letter to the complainants on 3 September, outlining the extraordinary stand-off.

The IPCC's letter said: "Despite the Metropolitan police's confirmation that they would comply with their statutory obligations in relation to investigating the reasons for stops under schedule 7, they have told us that they have completed their investigation into the complaints, but they claim that they cannot currently provide their findings to the IPCC. This is in our view incompatible with their statutory obligations.

"As a result, we instructed the Metropolitan police to provide us with the report for your complaint within seven days. The police have not complied, and so the IPCC is now in the process of making a legal claim."

Asif Bhayat from CagePrisoners said: "If the police have nothing to hide then they should come forward with their evidence to the IPCC. There appears to be an arrogance and high-handedness here. This attitude probably stems from the far-reaching powers given to the police under schedule 7 itself, which allows them to act with impunity in detaining large numbers of Muslims in the first place and then with further impunity in ignoring their own watchdog."

Schedule 7 allows police to stop people at ports and airports for up to nine hours without reasonable suspicion that they are involved in any crime. Failure to answer questions can be a criminal offence. Those stopped can also have their DNA taken and stored.

Keith Vaz, chair of the Commons home affairs committee, said: "It's very disappointing that this information is not being provided by the Metropolitan police to its own watchdog." Conservative MP David Davis, a former shadow home secretary, said: "That's astonishing. How on Earth can they justify that sort of behaviour by the Metropolitan police, unless they have something to hide."

Davis said the Home Office should instruct the Met to hand over its investigation to its watchdog rather than public money being spent on a court case: "This is potentially a very serious infringement of people's liberty and contrary to what we as a country stand for."

The Met said in a statement: "The MPS recognises the IPCC's role in scrutinising public complaints relating to schedule 7 stops and have been working hard to agree a procedure for dealing with such investigations that is acceptable to all stakeholders.

"We clearly set out our position to the IPCC and have offered to provide as much information about the investigations as possible. However, we have received further correspondence from the IPCC, which we are now considering."

IPCC deputy chair, Deborah Glass, said: "There is a statutory obligation to provide these findings to the IPCC. We believe the MPS are acting unlawfully and we are preparing to take the matter to the high court."

The IPCC said that concerns about schedule 7 led it in 2011 to decide to supervise complaints about its use. The watchdog said the Met had refused to investigate so it ordered the force to do so in February 2013. Two months later, having seen no evidence that it had started any investigation, the watchdog threatened legal action. The IPCC said the Met again promised to investigate but then said it would not give the watchdog the findings of its investigation.

People from ethnic minorities are up to 42 times more likely than white people to be the target of schedule 7. Some counter-terrorism officials fear the random nature of schedule 7 means a legal challenge could see the power struck down for being arbitrary. This is what eventually happened to the part of the act that allowed stop and search in the street without any suspicion, which was ruled unlawful by human rights judges.