David Miranda, the partner of Guardian journalist Glenn Greenwald, has instructed Bindmans and Matrix Chambers to advise him on potential legal action against the Home Office and the Metropolitan Police after he was detained for nine hours at Heathrow airport this weekend.

Miranda has instructed Bindmans associates Kate Goold and Gwendolen Morgan in relation to his recent detention under Schedule 7 of the Terrorism Act 2000. Bindmans has in turn instructed Matrix Chambers barristers Matthew Ryder QC and Edward Craven.

Bindmans this morning (20 August) sent a letter before action to Home Secretary Theresa May and Metropolitan Police commissioner Sir Bernard Hogan-Howe.

The letter states: "We write to inform you that we intend to challenge our client's detention under Schedule 7 Terrorism Act 2000 at Heathrow airport on 18 August 2013, and the consequent unlawful taking and retention of his property including sensitive journalistic materials.

"We require immediate undertakings, set out below, to prevent any further harm caused by the Defendants' actions whilst the legality of the seizure of his property is in the process of being determined. If undertakings in the following terms are not provided by 12pm on 20 August, we will be left with no option but to seek urgent interim injunctive relief in the High Court and seek costs on an indemnity basis.

"We ask that the Secretary of State and the Commissioner of Police of the Metropolis undertake that there will be no inspection, copying, disclosure, transfer, distribution or interference, in any way, with our client's data which was seized pursuant to Schedule 7, pending determination of our client's claim."

Writing on Twitter, The Guardian's director of editorial legal services Gill Phillips added that the newspaper was supportive of the action.

Miranda is the partner of Glenn Greenwald, The Guardian journalist who interviewed Edward Snowden, the former US spy agency contractor who has been granted asylum by Russia.

On Sunday (18 August) Miranda was held at London's Heathrow airport for nine hours while passing through en route to Rio de Janeiro.

The Bindmans letter confirms that Miranda's property included items whiched contained "sensitive, confidential journalistic material".

The letter continues: "If the police seek access to sensitive/personal/confidential/journalistic material of this kind they will ordinarily be required to do so by way of a production order and will have had to satisfy a number of important legal requirements that protect journalistic material of this nature.

"The use of Schedule 7 powers against our client appears to have enabled the police to circumvent all of those important legal protections and obtain that information from him by coercive means accompanied by the threat of prosecution if he failed to comply."

Speaking to Legal Week, Bindmans' Morgan said: "We have served a letter before action on the Secretary of State for the Home Office and the Met Police Commissioner threatening judicial review of this most egregious use of the Schedule 7 powers to obtain sensitive journalistic materials.

"We consider this exercise of the Schedule 7 power frustrates the legislative purpose of the Terrorism Act 2000. The action to detain and question our client for nine hours whilst he transited through the UK, and seize sensitive journalistic material, amounted to a disproportionate interference with our client's fundamental rights under Articles 5, 6, 8 and 10 of the European Convention on Human Rights.

"Section 7 is an incredibly Orwellian power and even the Government's Independent Reviewer of Terrorism Legislation has expressed his concerns over its use."

In a statement, Scotland Yard said Miranda's detention was "subject to a detailed decision making process".

"The procedure was reviewed throughout to ensure the examination was both necessary and proportionate. Our assessment is that the use of the power in this case was legally and procedurally sound. Contrary to some reports the man was offered legal representation while under examination and a solicitor attended," it added.