More than one million calls to the Universal Credit helpline have been abandoned without claimants getting an answer over a 12-month period, new figures show.
Labour MP Jim McMahon said the sheer number of calls going without an answer could mean many claimants were giving up trying to get through and not getting the support they needed.
However, the Department for Work and Pensions (DWP) called these claims "disingenuous", saying calls were answered on average within five minutes.
Figures obtained by Mr McMahon through a written parliamentary question show about 1.3 million calls were abandoned between September 2016 and October last year.
This is around 12% of all calls made to the helpline over that timeframe.
More than 120,000 calls were abandoned each month in August, September and October, the highest numbers over that period.
Mr McMahon cited reports that staff were struggling to cope with the sheer volume of calls, and that applicants and claimants were being forced to wait for lengthy periods of time to receive help.
"Everything we hear about Universal Credit suggests that it is failing in its purpose to provide help and support to vulnerable and low income claimants," he added.
"I worry that at this busy time of year, people needing help to make ends meet - many of whom are hardworking people - won't get the support they need from the Government."
In November, the Government scrapped charges for the helpline, after it was revealed that people could be paying up to 55p a minute for calls to fix problems with their claim.
The DWP says that 95% of Universal Credit claimants now claim online, with more than 80% also reporting any changes to their claim digitally.
However, Mr McMahon said there was no evidence to suggest that those abandoning calls were moving online.
"DWP have made a very broad assumption that abandoned calls are due to claimants moving online," he said.
"Not only is the minister's original answer entirely misleading, it raises a big question about how much of a handle the DWP have on the experience of their customers."
Calls to the Universal Credit helpline were answered within five minutes on average in December.
The calls per claim ratio for the Universal Credit full service has fallen from 2.7 in April 2016 to 1.0 by the end of October 2017.
A DWP spokeswoman said: "These claims are disingenuous - there are a number of reasons why someone might end their call, and calls to our UC service lines are being answered within five minutes."
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