Northern Ireland Secretary Brandon Lewis is to be challenged in court for “failing” to implement the Stormont House Agreement.

A lawyer acting for Thomas Braniff, whose father David was killed by loyalist paramilitaries in 1989, said the action is over proposals for dealing with the past in the 2014 accord – which have not been implemented.

The Braniff family say the case was not reviewed by the now defunct Historical Enquiries Team and has been in a queue for the PSNI’s Legacy Investigation Branch since 2015.

The family has also raised concerns about the possibility of collusion between the security forces and Loyalists in the attack.

They said the Police Ombudsman accepted a complaint about their case, but say it has not yet been assigned to an investigation.

The Stormont House Agreement included mechanisms such as a new independent body to take forward criminal investigations into outstanding Troubles-related deaths called the Historical Investigations Unit (HIU).

In recent weeks there have been reports that the UK Government is planning to introduce a form of amnesty for both former soldiers and paramilitaries.

Solicitor Setanta Marley, of the firm KRW Law, said the case affects not only the Braniff family, but “every single victim of the conflict”, adding that “judicial oversight is very much needed”.

“Judicial review proceedings have been lodged in the High Court on behalf our client, Thomas Braniff,” he said.

“Mr Braniff seeks to challenge the Secretary of State’s continuing failure to implement and give effect to the Stormont House Agreement and/or a continuing failure to implement a mechanism or mechanisms consistent with the principles of the Stormont House Agreement and in line with the government’s commitments in New Decade New Approach.

“Mr Braniff claims, inter alia, that the Secretary of State’s impugned decision is a clear departure from the unambiguous policy of the government as contained within the Stormont House Agreement and reaffirmed in NDNA and is therefore in contravention with Mr Braniff’s legitimate expectation that the terms of the Stormont House Agreement would be implemented within a reasonable time.

“Despite promise after promise, victims of the conflict have had to wait patiently for years for the British Government to introduce legislation to deal with the legacy of the conflict. Now it appears we are at the stage where legislation is forthcoming, and the British Government has broken those repeated promises by committing to legislation that drives a coach and horses through the Stormont House Agreement.

“This unilateral move by the British Government does not only impact upon my client and the Braniff family, it impacts upon every single victim of the conflict and judicial oversight is very much needed.”