Benefits claimants face "disaster" after it was revealed the Universal Credit helpline will be closed for most of Christmas.
Labour MP Frank Field, who chairs the House of Commons' Work and Pensions Committee, made the claims as the phone line became free to use on Wednesday.
He alleges the helpline will be fully open for just two days out of 10 – on 28 and 29 December.
But the Department of Work and Pensions pointed out a helpline in Scotland would also be open for use for claimants across the UK on 27 December, while also stressing the closures were usual practice for Bank Holidays.
Mr Field has written to the Prime Minister to warn that claimants "risk being exposed to destitution over Christmas" and urge her to avert a potential "disaster".
"The thought of the chaos this will cause to the lives of some of my poorest constituents, and those across the country, is a hugely troubling one," he told the PM.
The revelation comes weeks after the Department of Work and Pensions decided to eliminate the cost of calling the 0345 number after claimants complained it cost 55p a minute to call from mobiles and 13p a minute from landlines.
It was one of a series of changes following the mass introduction of the controversial benefit, including reducing the waiting time for first payments from six weeks to five.
Chancellor Philip Hammond also used last week's Budget to announce there will be more help for those facing rent arrears when changing over from existing benefits to Universal Credit.
On the Christmas closures, Work and Pensions Secretary David Gauke told Sky News: "We've got the usual operation. The hotline doesn't operate on Bank Holidays but that has been the case for previous Christmases without particular problems.
"Of course a lot of information is available online which has not been the case before, so if people want financial support in advance of Christmas then engage with your local JobCentre"
Charities and Labour say the impact of the delays in payment could mean a bottleneck of worried claimants left short of cash over the festive season.
There has also been criticism that the changes to ensure people are not left out of pocket are being phased in next spring, rather than immediately.
Mr Gauke told Sky News the delays were merely bureaucratic.
"Getting rid of waiting days, we have to pass a regulation through the House of Commons, so there is a legislative requirement there," he said.
"When it comes to the housing benefit transitional payment we have got to work with local authorities, so it can take a bit of time.
"It's operational, practical points we have to get through, before we can implement it, there's no other reason".
A DWP spokesperson said: "Just like organisations across the country, our offices are closed on public holidays but every year we bring forward over 20 million payments worth more than £4bn to benefit claimants.
"Phone lines are open on the 27 December to help people with any urgent payment related enquiries, and open as usual every other day around the Christmas period."