The use of remotely piloted drones by British forces in Afghanistan may be in breach of international law, a controversial legal opinion circulated to peace campaigners and released on Saturday claims.

The argument challenges the well established legal defence set out by the RAF for deployment of unmanned aerial vehicles (UAVs) in the UN-sanctioned conflict.

Publication of the document coincides with the court appearance this week of six anti-drone protesters who pleaded not guilty to causing criminal damage following the first mass trespass inside the RAF's new ground control for Afghan drone operations.

Written by Phil Shiner and Dan Carey of the Birmingham-based Public Interest Lawyers, the legal opinion argues that use of drones inside Afghanistan, which is a UN-declared conflict zone, is subject to the European convention on human rights (ECHR). That principle is already established in British case law, they say, in relation to the case of Al Skeini, which went to judges in Strasbourg and concerned the killing of civilians during British security operations in Iraq.

Their document states: "The requirement to use 'no more [force] than absolutely necessary' in article 2(2) [of the European convention relating to when it is permissible to take life] places a significant restriction on drone use.

"Only when it is absolutely necessary to kill someone rather than arrest/disable them will the use of drones be lawful. And even then, drones may only be used for one of the purposes in article 2(2), most relevantly, in self defence under 2(2)(c).

"Provided therefore that UK jurisdiction for the purposes of the ECHR is established, then the application of the ECHR would limit the use of drones solely to situations in which there is an immediate threat to life. This prevents the carrying out of 'targeted killings' and narrowly circumscribes their use even on 'the battlefield'."

"There is therefore a strong presumption that the UK's drones programme is in breach of international law."

The protesters, two of whom are priests, entered RAF Waddington in Lincolnshire on Monday and were brought before Lincoln magistrates on Tuesday. They say their action was to prevent crimes being committed in Afghanistan. A fence was cut, pictures of civilian victims distributed and a "peace garden" created inside the base.

The six Disarm the Drones activists are: Chris Cole, a drones researcher from Oxford; Fr Martin Newell, a Catholic priest from London; Rev Dr Keith Hebden, an Anglican vicar in Mansfield; Susan Clarkson, a Quaker pensioner from Oxford; Henrietta Cullinan, a teacher from London; and Penny Walker, who describes herself as a grandmother from Leicester.

Cole said: "We cut the fence in order to prevent more serious crimes in Afghanistan. We were inside for about an hour and a half before they arrested us."

Hebden told the Guardian: "Drones go against international law, you can't warn anybody or engage with the local community. There's between a one and four-second delay between people on the ground [in the UK] pulling the trigger and the drone firing. They are not the accurate weapon commonly portrayed."

Walker said: "This was the first mass trespass at RAF Waddington. Our defence is that we were preventing a worse crime."

All six were held in custody over Monday night. Several said their computers had been seized during police searches despite the fact that conspiracy charges had been dropped. A condition of their bail prevents them returning to Lincolnshire.

Remotely controlled armed drones used to target insurgents in Afghanistan have been operated from RAF Waddington, the home of XIII squadron, since April. The Ministry of Defence has consistently defended its use of remotely piloted drones and published detailed legal justifications. A spokesman said: "All operations, including those involving unmanned aerial systems, are informed by appropriate legal advice and are conducted in accordance with applicable International Humanitarian Law.

"Unmanned Air Systems are subject to legal reviews during the acquisition process, in accordance with the UK's responsibilities under article 36 of Protocol I Additional to the Geneva conventions of 1949. The reviews have concluded that UK UAS currently in use are capable of being used lawfully and in accordance with all relevant international and domestic law."

A defence source questioned whether the same conclusions would apply to weapons dropped from traditional aircraft or even artillery and mortar rounds.