Charities have welcomed a High Court judge's ruling that the Government took an "unlawful and unacceptably long time" to pay new welfare benefits to two disabled people.
The judge said the unnamed claimants were not to be treated as test cases.
But many organisations working with the vulnerable say thousands of others have suffered because of similar delays and may also have been treated unlawfully.
The claimants, Ms C and Mr W, had asked the judge to declare that Work and Pensions Secretary Iain Duncan Smith breached his common law and human rights duties to provide them with personal independence payments (Pips) within a reasonable time.
Mrs Justice Patterson, sitting in London, declared that the delay in both cases was "not only unacceptable, as conceded by the defendant, but was unlawful".
The judge had been told by Lisa Giovenetti QC that many disabled people had been forced to turn to loan sharks and food banks because it was taking "months and months and months" to get access to payments.
Pips are replacing the disability living allowance (DLA) in sweeping Government reforms of the benefits system. They are designed to help disabled adults meet the extra costs caused by disability.
The judge said in Ms C's case the delay was some 13 months, from September 9 2013 until the determination of her benefit on October 24 2014.
In Mr W's case the delay was some ten months, from February 3 2014 until December 2014.
The judge said both cases had called for "expeditious consideration" as the claimants suffered significant disabilities and were to be regarded as "the most vulnerable people in society."
She added: "There can be no public interest in delays such as was the case here."
Ms C, from Kent, has been diagnosed with ME and suffers from severe depression and other health problems.
She said: "I was completely isolated during the nine months I was waiting for my payments."
Gillian Guy, chief executive of Citizens Advice, said after the ruling: "People are turning to local Citizens Advice in their thousands as they are left high and dry without the financial support they need to live with their health condition or disability."
Justin Tomlinson, Minister for Disabled People, said "decisive action" had now been taken to speed up payments.
He said: "The average new Pip claimant now waits only seven weeks for an assessment.
In her ruling, the judge said it would be "inappropriate" to use the two cases as test cases "because of the considerable variations in individual circumstances".
She added: "I do not think it is the role of the court to give guidance in a situation which has been evolving and with which the defendant now appears to be grappling in a way which is entirely appropriate."
But Anne-Marie Irwin, the specialist public lawyer at Irwin Mitchell who acted for the disabled claimants, described the ruling as "a significant legal judgment" and said it "sends a clear message that the unacceptable delays faced by many people may also be unlawful".
Ms Irwin said: "A huge number of vulnerable people have been left in the lurch as a result of unacceptable flaws in the Pip system, with Public Accounts Committee chair Margaret Hodge in June last year, calling the issues 'nothing short of a fiasco'."
Food bank provider the Trussell Trust said delays to benefits payments were one of the major reasons for people seeking help.
Trust chairman Chris Mould said: "Many problems the Trussell Trust has seen in food banks in past years are related to appeals against Department for Work and Pensions decisions that disabled people were actually fit for work.
"In those circumstances people often wait months with their payments suspended whilst the review machinery grinds through its process."
Many other disability charities joined the chorus of criticism.
Mark Serwotka, general secretary of the Public and Commercial Services Union, said: "Cases like this expose Iain Duncan Smith's brutal, unforgiving and suspicious attitude to people who rely on social security and the failure of privatisation to offer the kind of help and support DWP staff want to provide."
Downing Street acknowledged there had been "problems" in the early phase of the introduction of Pips.
The Prime Minister's official spokeswoman said: "The Government has taken steps to address this, bringing in 800 more staff, increasing the number of health professionals working on it, and we are now starting to see progress."
The court judgment is the latest setback for the DWP, which has also suffered from problems over the introduction of the Universal Credit, but No 10 said David Cameron "fully supports" the approach they had taken.
The spokeswoman said: "The department has a wide range of challenging policies and objectives to deliver, many of which are complex, and the PM fully supports the approach they have taken to look at where they can learn lessons and make changes to make more progress.
"The measures that they have brought in on Pip mean that claimants are now only waiting seven weeks for an assessment and the backlog is coming down."