Senior Metropolitan police officers devised a policy of holding all people arrested on riot-related offences in custody and recommending that the courts also refuse bail after they were charged, according to a leaked "prisoner processing strategy" that lawyers argue could pave the way for a mass legal challenge.

The document, seen by the Guardian, was circulated to all investigating officers at the height of the violence two weeks ago by Operation Withern, the codename for Scotland Yard's emergency response to the outbreak of violence in the capital. It suggested that no one arrested in or after the riots should be let off with a caution – regardless of the offence – and that everyone arrested should be held in custody, with a recommendation that bail should also be denied when the case first goes to court.

Lawyers began proceedings on Monday for the first judicial review of the custody procedures, which resulted in 62% of those arrested for involvement in the riots remanded in custody compared with a normal rate of around 10% for more serious offences. They claimed the document amounted to a blanket policy of mass imprisonment of people.

The police document argues that the policy was necessary to prevent further public disorder as violence spread through the capital. But it also acknowledges that the force was so stretched at the height of the riots that it was "impractical" to bail people while they conducted "protracted" investigations, suggesting that investigating officers use special rules to fast-track cases to the courts with less evidence than is normally required. The recommendation could expose the Metropolitan police to accusations that it adopted a policy of "conveyer belt" justice in order to deal with its unprecedented workload.

The document, titled Operation Withern: prisoner processing strategy, includes a suggested statement for investigating officers to use in the prosecuting reports of individual cases, which are then passed to the Crown Prosecution Service. It says: "A strategic decision has been made by the MPS [Metropolitan Police Service] that in all cases an application will be made for remand in custody both at the police station, and later at court. This decision has been made in the interest of public safety and the prevention of further cases of disorder. The spontaneous nature of these offences and the significant burden it has placed on police resources has meant that not all inquiries have yet been completed. Some inquiries, such as gathering of CCTV, are not capable of being progressed at present due to the ongoing public disorder in and around London.

"As a result this case requires the application of a 'threshold test' for a charging decision based on the evidence present and the expectation that further evidence may be forthcoming."

Elsewhere the document says: "The volume of prisoners being processed makes it impractical to bail for the purpose of protracted investigation. Where evidence of an offence exists charging authority should be sought, that is likely to mean that the threshold test is applied."

The threshold test allows prosecutors to lower the burden of proof needed to remand someone in custody where there is reasonable suspicion and prospect of a conviction, and where there is a substantial risk if they are released.

The document sheds significant light on the Met's processes and could explain why people accused of apparently minor offences such as theft of small items or receipt of stolen goods were not cautioned. They included a 23-year old student with no previous convictions who was refused bail and then sentenced to six months in prison for stealing a £3.50 bottle of water. The debate about sentencing of people accused of taking part in the riots has so far focused on the courts' right to use "exemplary" sentencing – harsher sentences to deter people from rioting. But the document suggests that in deciding whether or not to grant bail the courts would have also been considering recommendations from the police to detain people in the vast majority of cases.

The document came into the hands of the solicitors Hodge, Jones & Allen, who have written to the Metropolitan police informing them they are starting judicial review proceedings of the decision not to bail an unnamed client, who was arrested for possession of £2,500 of items looted in the riots in south London.

The 25-year old care worker and mother of a two-year-old girl had no previous convictions and there was no evidence that she was involved in the looting.

Edward Kirton-Darling, the solicitor acting in the case, said: "The right to bail is a long-standing and essential part of our criminal justice system. It should be carefully considered and each case should be looked at on its own merits.

"In relation to the riots, it seems that the Metropolitan police took a strategic decision to apply a blanket ban and deny everyone bail, no matter what their circumstances. I consider this policy is unlawful as a result."

The lawyers' letter to the Met describes the policy as amounting to "unlawful arbitrary detention" of people. The existence of the policy has a "chilling" effect on Article 5 under the European court of human rights which guarantees an individual's liberty and security, it says. Adopting a pre-action protocol for judicial review, the letter demands an apology for the violation of the woman's fundamental rights. The Met said: "Guidance was issued to officers to ensure a consistent approach to an investigation which was, and remains, unprecedented in its volume and complexity.

"To ensure the interests of justice were served, prevent further disorder and protect the public it was made clear that a decision should be sought to charge where there was sufficient evidence. With courts sitting extended hours, the recommendation that those charged were remanded in custody was made to ensure cases were dealt with quickly and again to protect the public from potential further disorder.

"Cases were, and continue to be, looked at on the basis of the evidence available. Where the threshold to charge was not met people have been bailed to return pending further inquiries, released with no further action or - in a small number of cases - dealt with by other police disposals."