The Independent Police Complaints Commission (IPCC) said it had given the Metropolitan Police seven days to disclose its findings into outstanding complaints following its "continued refusal" to investigate.
The watchdog said it was supervising 18 investigations into the force's use of a part of the Terrorism Act 2000 that lets officers hold people for up to nine hours at ports and airports without reasonable suspicion of involvement in terrorism.
The measure has come under scrutiny following the detention at Heathrow airport of the partner of Guardian journalist Glenn Greenwald, who reported on documents leaked by NSA whistleblower Edward Snowden.
The IPCC said it told the Met in February to "investigate the rationale for stopping and questioning people under Schedule 7" of the Act.
It took two months and the threat of legal action before the Met agreed to look into the matter - but has since refused to tell the IPCC what it found.
An IPCC spokesman said it was "prepared to take the matter to court."
The watchdog is also supervising seven other probes into the misuse of anti-terror powers in Manchester, South Wales, Hampshire, Kent and Sussex.
One complaint against Greater Manchester Police was upheld after the use of firearms during a detention was ruled "excessive."
Scotland Yard claimed it was "working hard" to agree a procedure for dealing with investigations with the IPCC.