Liam Allan was put on trial on 12 charges of sexual violence because the police failed to hand over text messages that would have exonerated him

The scale of the failure by police and prosecutors to disclose vital evidence in criminal cases is exposed today in documents showing that such behaviour is routine and deliberate.

A dossier seen by The Times reveals a commonly held view that the defence is not entitled to see all the evidence. It discloses the tactics used to stop it being handed over, with officers in at least one force apparently trained in how to avoid making available material that might undermine their case.

The file draws on the reports of 14 focus groups with the police, and others with prosecutors and judges, as well as a survey of prosecutors.

The findings come after this newspaper reported on a series of rape cases that collapsed at