Police officers are trained to hide evidence that would undermine their investigations in court, a damning dossier reveals.

Documents show how police and prosecutors employ certain tactics to stop defence teams from seeing materials that could help their client during their trial.

Other accusations include officers being reluctant to record witness statements that hurt their case or presenting vital evidence to lawyers at the last second.

The revelations come after the recent collapse of several rape cases, leading to heightened concerns that evidence is not being disclosed properly.

The case against Liam Allan (pictured), who was accused of rape, was dropped days before he was due to stand trial after evidence including his accuser's diary was uncovered

One ploy used by police is to put evidence 'on the MG6D', which refers to a list of sensitive unused material which a defendant's legal team does not have access to.

Examples of legitimate materials which may be put on the MG6D include those relating to the interest of national security or information about an undercover officer.

According to The Times, the dossier is compiled from reports of 14 focus groups of police officers, judges and prosecutors.

One comment in the file read: 'In even quite serious cases, officers have admitted to deliberately withholding sensitive material from us and they frequently approach us only a week before trial.

'Officers are reluctant to investigate a defence or take statements that might assist the defence or undermine our case.'

Referring to the practice of exploiting the MG6D, an inspector said that officers are'trained to put items on there that they do not want disclosed to the defence'.

Scotland Yard commissioner Cressida Dick (shown) said officers must investigate rather than blindly believe allegations and should keep an open mind when a victim has come forward

The revealing dossier was obtained under a freedom of information act request by charity the Centre for Criminal Appeals.

More than 900 suspects had charges dropped last year because police and prosecutors failed to hand evidence to defence lawyers.

In the lead up to criminal trials, police and prosecutors have a duty to disclose evidence that might either assist the defence case or undermine the prosecution.

But the recent collapse of several rape cases has heightened concerns that evidence is not being disclosed early enough, and that the rules are not being followed.

The trial of student Liam Allan, 22, who was charged with six counts of rape, was halted by a judge after it emerged his accuser had sent hundreds of messages to friends that would have cleared him.

The case against an Oxford student accused of rape was dropped days before he was due to stand trial after evidence including his accuser's diary was uncovered.

Yesterday it was announced by Scotland Yard that officers would be abandoning the policy of automatically believing victims of rape.

Commissioner Cressida Dick said officers must investigate rather than blindly believe allegations, and should keep an open mind when a victim has come forward.