Justice secretary Kenneth Clarke has defended plans to extend secret trials across a range of proceedings in the civil courts, arguing that a ‘unique and unprecedented’ terrorist threat means that evidence affecting national security can be safely disclosed only behind closed doors.

A measure in the Queen’s speech on 9 May is expected to propose extending closed trials beyond terrorism to include civil cases involving inquests, prisons, the police and organised crime.

Clarke said ‘closed material procedures’ would allow ministers to defend themselves against damages claims by disclosing sensitive evidence without endangering the national interest or exposing vulnerable sources. Critics of the plans argue that closed trials lack transparency, are unfair and serve to build a body of secret case law to which only the government and a few others have access.

However, giving evidence before parliament’s joint committee on human rights on Tuesday, Clarke insisted that his prime interest is the integrity of the legal system.

‘Nobody is more committed to open justice than me,’ he said. ‘I’m not looking for special powers to enforce the Litter Act. The [2011 justice and security] green paper is a genuine consultation to discover how we can avoid cases, such as the Guantanamo Bay claims, where the government had to fold its tents and move away, rather than commit evidence that might have betrayed its spies and lost the co-operation of our allies.

‘In an ideal world every hearing would be perfectly open. But it only requires one case to go wrong to alert an al-Qaida cell and put the public and our sources in danger.’