Andrew Mitchell was repeatedly denied entry on his bicycle to Downing Street by armed police officers more than a year before the so-called Plebgate incident in September 2012, prompting a formal complaint by No 10 to the Met.

A "restricted" letter from the head of security in the prime minister's office to the Met's diplomatic protection group in June 2011, released by No 10, warned that there was "no just reason" why Mitchell should have been prevented from entering Downing Street on his bike.

"I am writing formally to complain about the conduct of your officers who manage access into the rear of Downing Street at D11," John Groves, the head of security and business continuity in Downing Street, wrote in a letter on 7 June 2011 to Inspector Ken Russell of the Met's diplomatic protection group (DPG). Groves added: "I cannot see any just reason why access was refused."

The letter was seized on by supporters of Mitchell who claimed that early evidence of police heavy-handedness – 15 months before the so-called Plebgate confrontation in Downing Street in September 2012 – highlighted a pattern of behaviour and showed that Met officers had been spoiling for a fight.

Mitchell was forced to resign as chief whip in October 2012 a month after police officers claimed he had described them as "fucking plebs" when they refused to allow him to cycle through the main gates at Downing Street. The former minister, who reluctantly wheeled his bike through the side gates, admitted swearing in the presence of Toby Rowland, one of the officers on guard, who is now suing him for libel. But Mitchell strenuously denies having described police as "plebs".

Dominic Raab, the Conservative MP for Esher and Walton, said: "The more we learn, the more there appears to have been a concerted effort over months to disrupt cabinet ministers entering Downing Street."

The No 10 letter was sent to the Met in June 2011 after what was described as a "handful of incidents" in which cabinet ministers, including Mitchell, had been denied access through the back entrance to Downing Street on Horse Guards Parade or had not been identified within a reasonable period of time.

Groves wrote in his letter to the Met: "This morning there were two further incidences where DPG officers refused access to a cabinet minister who was here to attend cabinet. In one incident Andrew Mitchell was, again, stopped from entering the L-shaped road [at the back of Downing Street] on his bicycle. According to this cabinet minister the police officer [name redacted] said that he did recognise who he [Andrew Mitchell] was but would not let him. No clear reason was given. You will no doubt wish to check your officer's account of this exchange but I cannot see any just reason why access was refused."

The incidents took place at the rear entrance to Downing Street in the early summer of 2011 when Mitchell was international development secretary, in contrast to the Plebgate incident which took place at the front gates in September 2012, by which time he was chief whip. But the Downing Street head of security said that cabinet ministers should be allowed unfettered access to both entrances to Downing Street.

Groves wrote to the Met: "As far as I am concerned, members of HM cabinet are entitled to unfettered access to Downing Street any time of day or night and at any entry point. Although they do not have a Downing Street photopass, their access should be facilitated in an efficient and expedient manner as is possible; not least as they are here to see the prime minister. I think it is entirely reasonable that your officers should be able to recognise the relatively small number who come in on foot or by vehicle without the aid of a protection team."

The release of the letter by No 10 follows the disclosure in the Sunday Times over the weekend that police officers agreed on the eve of the Plebgate incident to prevent Mitchell from cycling through the main gates. In an email to his superior on 18 September a DPG officer asked "for backing from his superiors when DPG officers were going to refuse him [permission] to use the main gates on future occasions, as was bound to happen". The email features in a lengthy dossier submitted by Mitchell to the high court for the libel action by Rowland, extracts of which were published in the Sunday Times, which also published extracts from the Downing Street letter.

The letter is likely to raise questions about the investigation by the cabinet secretary Sir Jeremy Heywood into the Downing Street confrontation. Heywood was criticised by some Tories after saying that he only examined CCTV footage of the incident and emails from an off-duty police officer who later admitted that he had falsely claimed to have witnessed the incident.

The letter will also raise questions about the claim by the Met that a legal order from 2008 meant that "no one may use the street unless they are authorised or directed by a police officer". This was cited by Alison Saunders, the director of public prosecutions, in the CPS decision on the incident last November.

A CPS spokesperson said: "Police policy concerning use of the gates was considered by the CPS as part of the evidence provided by investigators in Operation Alice. Any questions on this policy itself should be directed to the police."

Sir Richard Ottaway, the Tory MP for Croydon South who is an ally of Mitchell's, said: "It is clear that there was a problem for a long time and it adds weight to the argument that this was a contrived row by the police. The best thing that could happen is that the Police Federation drop this libel action [by Toby Rowland] and bring the whole matter to a close."