Sir Keir Starmer is "watering down" benefits checks amid a record backlog of mental health cases.
The welfare assessment system has been at risk of "falling over", with an immediate need to make changes.
Pressures on the welfare system have been exacerbated by a "significant and continuing rise" in new Personal Independence Payment (PIP) claimants.
New rules, coming into force this week, will allow benefits claimants aged 25 and over to receive PIP awards for four years after an initial assessment, and a further six years following a review.
The number of people claiming PIP has rose to a record 3.9 million, costing the taxpayer £26billion a year, forecast to rise to £41billion by the end of the decade.
Labour has said widening the period between checks would save money, but officials have privately admitted the law was being changed to manage the welfare backlog, it is understood.
Helen Whately, the Shadow Work and Pensions Secretary, has warned that longer periods between checks will come at greater cost to the taxpayer.
She told The Telegraph: "Reviews are the only way we can check whether an award is still correct. Fewer reviews mean more people receiving handouts for longer, at greater cost to the taxpayer.
Helen Whately warned fewer reviews would come at 'greater cost to the taxpayer'
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"People who could work will instead be left on payments for years without anyone asking whether that is right for them or fair to the taxpayer.
"Instead of gripping the crisis, Labour is watering down the checks that decide whether awards are fair."
The Social Security Advisory Committee, the Government's independent welfare watchdog, raised several concerns about the decision to widen the gap between eligibility checks.
In a meeting earlier this month, officials said the policy was necessary because "the central concern is that the assessment system will 'fall over' if capacity pressures are not addressed".
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Senior DWP officials said there was an 'immediate need to act' while long-term reforms to the welfare system were being developed
| PAGovernment officials also declared there was an "immediate need to act" while long-term reforms to the welfare system were being developed.
Senior officials from the Department of Work and Pensions claimed extending the length of PIP awards would free up capacity and lead to more face-to-face assessments.
Just 49,000 PIP assessments were carried out in person between July 2024 and July 2025, dwarfed by the 1.1 million conducted online.
There has been a surge in PIP claimants for mental health conditions including anxiety, depression and ADHD.
The Government has claimed the reforms will save nearly £2million for the taxpayer
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Psychiatric disorders make up 39 per cent of all PIP claims, making it the single largest category.
Eduin Latimer, senior research economist at the Institute for Fiscal Studies, said a failure to reform the PIP system would lead to "harder choices", which could include "cutting other things", raising taxes or going against Labour's fiscal rules.
A Department for Work and Pensions spokesman said: "We inherited a broken welfare system with significant backlogs and contracts signed by the previous government requiring assessments to be delivered virtually.
"As a result of changes announced in the Budget, we will ensure health professionals’ time is spent where it is needed, on increasing the number of face-to-face assessments and Work Capability Assessments.
"These changes will help save the UK taxpayer £1.9bn over the Parliament and allow us to deliver personalised employment support to sick and disabled people who are able to work."