The High Court agreed with two unnamed disabled people who said it was unlawful that they had to wait 10 months to receive the new personal independence payments (Pips).
Livability, the UK's largest Christian disability charity, welcomed the ruling and agreed that an extended wait was "unacceptable" because it put a "strain and extra cost" on people affected.
Director of Operations, Liz Mell, said: "Any wait at all is obviously a problem for people but we're also realistic and understand that all processes take time but a wait of 20 or more weeks for such a large number of people is still unacceptable."
She added that the charity was "really hoping" it would force the government to change processes so that benefits can be paid quicker.
Mrs Justice Patterson said the two claimants, Ms C and Mr W, were the victim of a breach of common law and human rights duties by Work and Pensions Secretary Iain Duncan Smith.
She said the delay was "not only unacceptable, as conceded by the defendant, but was unlawful".
"They were each to be regarded as the most vulnerable people in society," she added.
"There is a high duty on local authorities to act promptly, consistently and appropriately to recognise social welfare benefits.
"There can be no public interest in delays such as was the case here."
The personal independence payments are replacing the disability living allowance (DLA) as the government reforms the benefits system.
They are paid to help disabled adults meet the extra costs caused by disability.
Chris Mould, Chairman of the Trussell Trust, a Christian organisation providing food banks, said: "Referrals due to benefit sanctions, changes and delays was the single biggest reason for people needing help from our food banks in 2014/15.
"For those disabled people attending our food banks affected by Personal Independence Payments this ruling at last provides some hope that changes might be made to avoid such unnecessarily long delays in future.
"We are pleased to have something definitive said about this issue; we have been speaking out for years about the difficulties these delays can throw people into.
"We now hope the Department for Work and Pensions will make the necessary changes to ensure no-one on, or eligible for, disability benefit goes needlessly hungry."
The Department for Work and Pensions said: "Successful Pip claims are always backdated to the date of the claim, meaning that no-one who has experienced a delay should suffer financial loss as a result."
The Prime Minister's 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."
Liz Mell speaking to Premier's Antony Bushfield: