An independent watchdog is to investigate a police force over the collapse of a trial of four men accused of the gang rape of a woman at a leading university.

Thady Duff, Leo Mahon and Patrick Foster, all 22, and James Martin, 20, were accused of raping the woman at a summer ball at the Royal Agricultural University in Cirencester, Gloucestershire.

She claimed she had been subjected to violence, including strangulation, on the night of the alleged attack in May 2014 and that some of it had been filmed and posted on Snapchat.

The trial had been due to begin at Gloucester Crown Court in March but, following delays due to the late disclosure of evidence and a review of the case, the prosecution offered no evidence and the four defendants were cleared.

Cleared: Thady Duff, Leo Mahon and Patrick Foster, all 22, (pictured) were accused along with James Martin, 20, of raping a woman at the Royal Agricultural University in Cirencester. But the trial against them collapsed

Last week during a hearing on defence costs, Judge Jamie Tabor QC, the Recorder of Gloucester, criticised Detective Constable Ben Lewis, the lead officer.

The judge said Det Con Lewis had got too close to the complainant and did not understand his job properly, which led to 'stark and very serious omissions' by the officer in failing to disclose 'game-changing' evidence.

The Independent Police Complaints Commission has decided an independent investigation is 'appropriate'.

'The circumstances of a criminal investigation conducted by Gloucestershire Constabulary, which was unable to proceed to trial due to issues raised during legal proceedings, will be examined by the Independent Police Complaints Commission,' the IPCC said in a statement.

'Issues have been raised concerning the police investigation and the disclosure of evidence in preparation for the trial.

'The matter was referred to the IPCC and we have decided an independent investigation is appropriate.

Thady Duff, Leo Mahon and Patrick Foster, all 22, (pictured) and James Martin, 20, had denied charges of rape and sexual assault

'The IPCC will carefully examine the actions of the officers involved during both the investigation and the pre-trial process.'

The quartet of students said last month that police treated them as ‘guilty until proven innocent’.

Detectives were accused of ‘cherry-picking’ evidence to support their case, while ‘airbrushing’ out anything that suggested the men were innocent.

The Gloucester Crown Court case against the four students fell apart after it emerged that the alleged victim had given ‘different accounts’ as a witness in another rape case.

Officers are also facing questions over why it took 13 months to charge the men, with lawyers alleging evidence had been ‘withheld’ by officers before the trial.

This included messages taken from the complainant’s phone hinting that she may have consented.

Meanwhile, last month it was revealed that the police officer accused of 'vandalising' the rape case against the four students is still working on serious crimes and had not been disciplined.

Gloucestershire Detective Constable Ben Lewis was accused of having 'broken' the trial process after holding back information that led to the friends being cleared of sex crimes.

But it emerged last month that DC Lewis, who it was claimed had flirted with the alleged victim and even held a formal interview with the girl in her bedroom before claiming the tape recorder had broken down, was still on active duty.

Gloucestershire Police had said last month that it was not going to be referred to the Independent Police Complaints Commission.

The rape case collapsed on the day it was revealed that the Crown Prosecution Service (CPS) last year dropped more cases than in any of the previous five years.

The damning figures revealed that one in every eight Crown Court cases – more than 12,600 – were abandoned before they had even started.

The group were accused of raping the alleged victim at the end-of-year ball at the Royal Agricultural University in Cirencester, Gloucestershire (pictured)

The young men were arrested on suspicion of rape and sexual assault after the drunken sex session on the night of the ‘Mad Hatters’ May ball at the Royal Agricultural University in Cirencester, Gloucestershire, in 2014.

A ‘pornographic’ video of the act was shared on social messaging app Snapchat – leading the woman involved to tell police she had been raped.

The young men were facing prison sentences of more than ten years if they had been convicted.

ROYAL AGRICULTURAL UNIVERSITY RAPE TRIAL: A TIMELINE From left: Leo Mahan, Patrick Foster, Thady Duff and James Martin - their defence barristers have now called for action following the trial OCTOBER 2013: Thady Duff was accused of a further sexual assault involving the same alleged victim, which allegedly took place in October 2013. MAY 2014: Leo Mahon, 22, Patrick Foster, 22, James Martin, 20, and Thady Duff, 22, had been accused of subjecting a woman to a rape ordeal on the night of the annual May Ball in 2014. JULY 2015: All four men were charged by the Crown Prosecution Service in July 2015. MARCH 29, 2016: A jury of six men and six women were sworn in on March 29 to hear the case but were discharged a week later not having heard any evidence. The group appeared at Gloucester Crown Court on March 29 to deny all charges against them. MARCH 30, 2016: Their trial had been due to start on March 30, 2016. APRIL 2016: The case had been reviewed last week and a decision not to offer any evidence against the four defendants had been made. APRIL 11, 2016: All charges were dropped save for one of possession of extreme pornography against Mr Duff. Advertisement

The Gloucester Crown Court case against the four students fell apart after it emerged that material was found on the alleged victim’s mobile phone about her sex life, including a ‘three-in-a-bed’ incident involving a soldier at a Wiltshire barracks.

Five months after the Royal Agricultural University's May Ball, where she said she was raped by the four young men, the woman was involved in a sex session with an Army officer which led to the soldier being accused of rape by another woman.

It is understood the alleged victim in the May Ball case had had sex with the soldier after a visit to an Army barracks. He was court martialled but cleared of rape and sexual assault charges after she gave 'different accounts' of the alleged rape.

The officer on the May Ball case was revealed to have kept quiet about the 'different accounts' the woman had provided.

Defence barristers acting for the four men argued the case showed the woman's interest in group sex and demanded to know why neither they, nor the Crown Prosecution Service, had been told about it.

Further analysis of the complainant's phone revealed she had sent text messages on the night of the ball which raised further doubts about her accusations.

After the case against him collapsed amateur jockey James Martin criticised the ‘devastating’ police investigation.

He said: ‘If they had done their job properly it would have been over a long time ago and I would have years of my life back.

‘It was hard, very hard. It’s always been there. It’s changed the way I think about things. I look at people in a different way now – a bit paranoid. It’s harder to trust anybody.

‘It has been hard, really hard for the families. It is a big relief for everyone. I’m relieved but annoyed it got this far'.

His barrister Edward Henry accused officers of ‘airbrushing’ and ‘cherry-picking’ evidence and said there will need to be a review, adding: ‘We need to know the answers to some questions. Why this should have gone on for so long as it has?

'Why it took 13 months to decide to charge these defendants in the first place?

Mr Henry had told the court that there had been an ‘absolute failure’ by police officers ‘to take notes except for self-serving acts on occasion’.

He said: ‘There are two notes where sexual behaviour has been mentioned to the officer and these notes have never made their way into the defence material. He has vandalised the trial process. It is broken and cannot be fixed'.