The UK is a State Party to both the Chemical Weapons Convention (CWC), and the Biological and Toxin Weapons Convention (BTWC). In accordance with these conventions, the UK maintains small quantities of chemical and biological warfare agents for research purposes, that are not prohibited under the conventions, to ensure that UK forces are adequately protected in the event that chemical or biological weapons are used against them.
The CWC categorises chemical warfare agents under Schedule 1 chemicals. Under the CWC the UK is permitted to hold a maximum aggregate quantity of one tonne of Schedule 1 chemicals. There are no set limits on quantities of individual chemicals within this overall amount. The MOD holds some Schedule 1 chemicals within the one tonne limit. The Schedule 1 chemicals held by MOD are between approximately 10 and 40 years old, although on occasion small quantities (typically a few grammes) of Schedule 1 chemicals that are not normally held have been manufactured. These holdings are inspected by the Organisation for the Prohibition of Chemical Weapons (the OPCW), the international body set up to implement the CWC, to ensure compliance with the convention.
The BTWC does not stipulate specific agents that are prohibited, nor does it specify quantities of agents that are permitted to be held. It permits the possession of biological agents and toxins of types and in quantities consistent with prophylactic, protective or other peaceful purposes, recognising that such agents exist in nature and are used for legitimate purposes, including the development of protective materials and medical countermeasures to biological agents and toxins.