A new system of disability benefits has begun to take effect across England, Scotland and Wales.
Personal Independence Payments (PIPs) are replacing Disability Living Allowance (DLA) as part of the government's welfare reforms.
The government claims PIPs will target resources more effectively towards those who need it most.
But a charity warns that almost a fifth of claimants - 600,000 people - could eventually lose their benefits.
Since April, thousands of new claimants in the north of England have already applied for PIPs. Now, new claimants of working age in the rest of Britain will also have to apply for a PIP, rather than DLA.
Northern Ireland will join the system later.
From October, PIPs will be extended further, when the government will start to re-assess existing claimants, but only those whose circumstances have changed.
The vast majority of the 3.2 million people who currently claim DLA will not be re-assessed until 2015 or later.
Many disabled people fear having to be put through the new in-person tests to assess their eligibility.
Phil Sumner, a former postman who has multiple sclerosis, told the BBC that the prospect of an interview is "quite intimidating".
"It's like being judged again. Filling out forms is bad enough. Face-to-face, I don't like anyway," he said.
But Lee Healey, managing director of benefits advisers IncomeMAX, said people who were eligible should not to be put off claiming their entitlement.
"There will still be a lot of people who can claim benefit for mobility and care needs," he said.
Figures from the Department of Work and Pensions (DWP) suggest that 450,000 will no longer be able to claim the benefit by 2018.
But the disability charity Scope said that - including those who would have been new claimants between now to 2018 - some 607,000 people will miss out in total.
The government claims the new system will better target those who need help.
Under DLA, most people filled in their own application forms, and did not have to re-apply, even if their health improved, the government said.
"Seventy-one percent would have indefinite awards, without regular checks," the disabilities minister, Esther McVey, told the BBC.
"So this is about targeting billions of pounds a year at the people who need it most."
However Scope believes the main motive is to spend less.
"Disabled people believe this reform is an excuse to save money," said Richard Hawkes, Scope's chief executive.
"It doesn't help that the minister is able to predict exactly how many disabled people will receive support before they have even been tested," he said.
Expenditure on DLA has gone up by 32% in the last 10 years.
But the government insists the introduction of PIPs is not about saving money.
Expenditure is still expected to rise from £12.6bn in 2009/10, to £13.8bn in 2015/16.
However, with 450,000 fewer people expected to receive the benefit by 2018, the cost to the taxpayer will be much smaller than it otherwise would have been.
Campaigner Kaliya Franklin, who receives the benefit, questioned the value of replacing the DLA system.
Most claimants understood the DLA system, she said, and people and their disabilities were often complicated, so it was not an easy system to simplify.
Under the new assessment system, 75% of applicants will be required to attend face-to-face interviews.
Those interviews will look at people's ability to wash, dress, cook and make journeys. But they will also assess reading and verbal communication skills. The government says they will therefore test mental, as well as physical health.
But Scope says the planned test is "deeply flawed."
Scope claims it will be a "tickbox-style medical assessment", which will not achieve the desired objective. It is worried that there could be a repeat of problems that occurred with the fitness-to-work test, known as the Work Capability Assessment.
Thousands have appealed successfully against their adjudications in such cases.
But Capita, one of the firms carrying out the assessments, has tried to ease fears about the tests. "It is more of an interview than a medical assessment," said Stephen Duckworth, the head of Capita's PIP programme.
"Applicants could be asked to bend over and touch their toes, but it will not be more complicated than that," he told the BBC.
He added that many new claimants actually understated their disabilities, so an assessment would help them. There was some "unnecessary concern" about the changes, he said.
An independent review will examine the new PIP payments in 2014, before most people are reassessed for the benefit.