The Government is going to review all 1.6 million claims for a disability benefit after it decided not to contest a High Court ruling.
Ministers previously said accepting the decision over claims by those with mental health conditions could see up to 220,000 personal independence payment (PIP) claimants awarded higher payments.
In a written parliamentary question, shadow work and pensions secretary Debbie Abrahams asked if there was a timetable for when claimants will be told if they will get their payments backdated.
Disabilities minister Sarah Newton said: "We are working with stakeholders to change the PIP assessment guide so that we can implement the judgment.
"Once we have completed this exercise we will be carrying out an administrative exercise to review cases that may be eligible and ensure that claimants receive the correct award.
"This will be a complex exercise and of considerable scale, as we will be reconsidering approximately 1.6 million claims.
"Whilst we will be working at pace to complete this exercise it is important that we get it right."
The changes will not affect all PIP claims but they will all be reviewed as an administrative exercise.
Ms Abrahams said: "Today's admission that the department will have to reconsider 1.6 million PIP claims to ensure that all claimants receive the correct award is shocking.
"The disabilities minister has refused to publish a timetable of how many months or even years it will take for this 'complex exercise' to be completed.
"The Government was wrong to cut PIP benefits in the first place, wrong to bring in the PIP regulations last year and it was wrong to repeatedly ignore the views of the courts."
Regulations introduced in March prevented an award of the higher PIP mobility rate if someone cannot take a familiar journey on their own unless it is "for reasons other than psychological distress".
December's judgments said the policy had been "blatantly discriminatory" against people with mental health conditions.
The changes in March overruled the decision of an independent tribunal in November 2016.
At the time, ministers said following the tribunal's judgment would affect 164,000 people's benefits and cost £3.7 billion extra by 2022.
Earlier this month Work and Pensions Secretary Esther McVey confirmed she would not appeal against the High Court verdict, and instead would implement the original tribunal decision in full.
Payments will be backdated to the original tribunal decision in November 2016.
Philip Connolly, policy manager at Disability Rights UK, said: "Many disabled people have lost out because of (the) changeover from DLA (disability living allowance) to PIP, and we welcome the announcement that the Government is going to review 1.6 million cases.
"We hope this doesn't mean that some disabled people are going to have to attend yet more assessments.
"This review highlights the ongoing and persistent failures of the assessment process, which is badly designed and implemented.
"Huge amounts of taxpayers' money is being wasted on poor quality assessments which deny disabled people benefits that they qualify for - that's one of the reasons the success rate at appeal is so high.
"We urge all disabled people who are turned down for benefits they believe they should get to use the independent appeals process if their claim is turned down."