The world’s chemical weapons watchdog is to hold a special two-day session in late June in response to Britain’s call to hand the body new powers to attribute responsibility for chemical weapons attacks.

The Hague-based watchdog, known as the Organisation for the Prohibition of Chemical Weapons, has until recently been seen as a scientific technical backwater, but as the controversy over the use of the weapons has grown, the OPCW has found its methods under attack from Russia and other supporters of the Syrian regime.

British ministers have accused Russia of blocking the UN from blaming the Syrian government for repeated chemical attacks on its citizens.

In a speech at the Chatham House thinktank last month, the OPCW’s director general, Ahmet Üzümcü, called for his organisation to be given the ability to identify the individual, group or country behind chemical attacks, saying the international community needed to address the gap.

At a meeting this week, Britain won the support of 64 OPCW member states for the special session, and the OPCW confirmed in a statement on Tuesday that this would now go ahead on 26 and 27 June.

Britain’s national security adviser, Mark Sedwill, said: “We recognise that the global norm against chemical weapons use is being threatened.”

British support for a strengthened role for the OPCW grew after the UK concluded that a Russian-made military nerve agent was used in Salisbury in March in the poisoning of the former Russian double-agent Sergei Skripal and his daughter.

The Salisbury incident followed an impasse in November last year at the UN security council when Russia blocked the renewal of the mandate for the body, known as the joint investigative mechanism, responsible for attributing chemical weapons attacks to groups or countries. Russia claimed the body’s procedures, including the chain of command over samples, was too lax.

Russia has also mounted attacks on the technical methods of the OPCW’s investigations into the Salisbury attack.

Speaking at the thinktank, Üzümcü said: “Today there might be good reasons actually to clarify the role of the OPCW itself in terms of attribution once it has the necessary information at its disposal. Wilful defiance of a valued norm should not be allowed to go unchallenged.”

He told the Guardian: “I don’t think the international community can afford to continue without an attribution mechanism to identify perpetrators of the use of chemical weapons. If accountability is avoided the potential acceptance of the use of chemical weapons as weapons of war and terror will not be deterred.”

An OPCW fact-finding team is due to report on whether chemical weapons were used in an attack in rebel-held Douma in Damascus. The attack led to US, French and British reprisals, including cruise missile strikes on alleged Syrian chemical weapons sites.