Asylum seekers who were subjected to DNA swabs to prove their origins may be able to sue the government after the Home Office admitted the lawful basis for taking those samples was “dubious”.

The admission emerged during a legal challenge brought by an unaccompanied child asylum seeker stranded in France who was desperate to be reunited with his older brother, a refugee in the UK.

He wanted to use independent DNA testing to prove he and his brother were blood relations. The brothers cannot be identified for legal reasons.

The upper tribunal of the immigration and asylum chamber found that the Home Office had failed to comply with its duties after a request from the French government, known as a take charge request, to consider whether the boy, now 17, is the younger brother of the man granted refugee status in the UK.

The Home Office had refused his request for family reunification under regulations known as Dublin III, believing the two males not to be blood relations.

The judges found the two are brothers and that as part of the take charge request, the Home Office should investigate the possibility of DNA testing for the boy in France or alternatively should consider allowing him to come to the UK where a voluntary DNA test could be carried out.

The boy argued that as Home Office officials took DNA swabs from asylum seekers in a 2009 pilot scheme, he too should be able to have a DNA test in areas subject to UK immigration law in France: Calais and Dunkirk. The Home Office argued that it had no power in law to take DNA samples and that the lawful basis for the 2009 scheme was dubious. The scheme is thought to have operated until March 2011.

Thursday’s judgment states: “However, having sought instructions, Mr Lewis [Home Office counsel] informed us that the lawful basis of the pilot was dubious and that the respondent was unable to identify the particular legal power that enabled immigration officers to take DNA samples under the scheme.”

Based on this admission from the Home Office, the boy’s solicitor, Jane Ryan, from the law firm Bhatt Murphy, called for an investigation into the Home Office’s apparently unlawful DNA testing of asylum seekers. “The Home Office needs to come clean about this suspect scheme and fully investigate its apparent abuse of power,” she said.

She added that an investigation should determine how long the Home Office has been aware of its apparently unlawful conduct, the number of people affected by it and confirmation that those affected would be notified of any claim they might have against the Home Office.

Ryan said DNA testing of asylum seekers as evidence of familial links was not wrong, but that it needed to be conducted in a proper, independent and professional way by doctors, not by Home Office officials.

Home Office justification at the time, according to its guidance, stated: “There may be a significant percentage of asylum applicants claiming to originate from a country which is not their own in order to further their asylum claim by claiming to be a different nationality.”

The tribunal also found that the court, not only the Home Office, has the power to decide if children are telling the truth about having relatives in the UK with whom they want to be reunited.

The two brothers hope they can be reunited in the UK very soon. The court ordered that the home secretary make a new decision on the take charge request by 3 August.

A Home Office spokesperson said: “We have noted the judgment [of the tribunal] and are considering our position.”

