Claims by a charity that d elayed assessments for a disability benefit have left at least 4,500 cancer patients waiting six months or more have been questioned by welfare officials.
The Department for Work and Pensions (DWP) said results of a survey should "at best be treated with extreme caution" because it involved only 210 patients.
Official statistics on waiting times are still being compiled.
Macmillan Cancer Support said its research exposed the "shattering" impact of problems with the introduction of Personal Independence Payments (PIP).
One in three of those asked said they had suffered anxiety, depression and other mental problems as a result, with more than half (56%) also reporting a financial impact - prompting fresh calls for urgent Government action to speed up the process.
As many as 2% had given up on their application altogether, Macmillan said.
The charity said it wanted to see waits reduced to the 11-week average for a decision on eligibility for Disability Living Allowance (DLA), which PIP has replaced, to help meet extra costs arising from a long-term health condition or disability.
But a DWP spokesman said: "Macmillan's report is based on a very small sample size using simplistic calculations to produce results, which at best should be treated with extreme caution.
?"Claims for terminally ill people are fast-tracked using 'special rules' under Personal Independence Payment and statistics show over 99% of people with terminal illnesses who have applied have been awarded the benefit. That means over 10,000 terminally ill claimants are now receiving PIP.
?"We have been working with Macmillan and they have acknowledged that improvements to the system have already been made.
"PIP replaces the outdated Disability Living Allowance which was introduced over 20 years ago. It includes a new face-to-face assessment and regular reviews - something missing from the old system - to make sure support is better targeted at those who need it most."
Most people applying for PIP undergo a face-to-face assessment to determine eligibility, which is carried out by private contractors, but a Commons committee has criticised "unacceptable" delays.
Some individuals with terminal conditions have died during the wait.
The survey found a quarter of those who have started their claim - which can only be made after three months of starting to suffer problems with day-to-day activities - are currently stuck in the system.
Jodie Patten, 31, from London, said she applied a month after being diagnosed with breast and bone cancer in October but is s till to be assessed.
"I have had to constantly call up to find out what is happening with my application. I have worked all my life, paid my taxes, and it feels like I'm begging for money," she said.
"My finances have been severely affected while waiting, day-to-day expenses such as food become more difficult to cope with. I already have to worry about cancer, and I don't need to worry about paying the bills as well."
The charity said 47% of patients were dissatisfied with the process - a third because of the delays and almost a quarter (23%) because of "poor communication from the Department for Work and Pensions (DWP)".
Macmillan head of policy Duleep Allirajah said: "Our report shows the real and shattering impact of these PIP delays are having on cancer patients.
"It is unacceptable that people struggle to heat their homes, are saddled with debt or are left anxious or depressed because they are waiting so long for their much-needed benefits.
"These delays are a further blow to cancer patients who have to prove that they have been affected by their disease for at least three months before the state will consider them as eligible for help.
"The Government has a duty to ensure that the new disability benefit works at least as well as the old one and Macmillan is calling on them reduce waiting times to 11 weeks as a matter of urgency.
"We have raised these issues with the Department for Work and Pensions and are keen to continue to work with them to help improve the benefits claiming process for cancer patients."