Thousands of people could be plunged into debt ahead of Christmas because of universal credit delays, a housing association body has said.
People applying for the payment this week - which is now given instead of six benefits - face a five week wait.
Community Housing Cymru said 84% of tenants owe an average £556 on rent and the wait would "plough them into debt".
A Department for Work and Pensions (DWP) spokesman urged people to "apply without delay".
CHC, the membership body for housing associations in Wales, said it believed the wait would mean the money will not arrive before Christmas.
It said this would create "significant hardship" for many people facing shortfalls in income.
Universal credit was launched in 2010 to make the benefits system simpler, by giving one payment instead of six for things such as housing benefit, child tax credit and working tax credit.
But many of the UK-wide 2.3 million recipients have complained about issues such as receiving less money.
Universal credit 'destroyed part of my life'
Mother-of-five Gina Francis, from Talbot Green, Rhondda Cynon Taff, had just secured a flat in the town last year through Rhondda Housing Association, but said she was given an unwelcome surprise.
"I was quite excited at the thought of moving on the same day that I had to go to the Job Centre to apply for Universal Credit," she said.
"I didn't know that you had to go without money for six weeks until I go there so... to have no money was devastating, coming up to Christmas and I couldn't buy my children any presents.
"I didn't want to go on with life because what was the point? It's Christmas as well. I borrowed £20 to buy some microwavable meals.
"It affected me for a long time and I had arrears with my rent, arrears with the gas, electric, everything.
"This Universal Credit did nothing for me. I don't think it's a good thing. It affected my mental health."
Despite improvements to the system, CHC said the £556 rent debt was almost double that owed by tenants under the old housing benefit system.
This means thousands of families could be facing Christmas without the luxuries many people take for granted - turkeys, mince pies and presents.
"Despite some improvements to the universal credit system, the increase in food bank usage and surge in rent arrears shows it is still causing unnecessary hardship for people," said CHC's Will Atkinson.
He said staff were now spending two to six hours a day supporting tenants with related issues, calling the policy "not yet fit for purpose".
Mr Atkinson said welfare needed to be a priority for parties campaigning ahead of next month's general election.
"We are now calling on the next UK government to recognise the impact universal credit has had by ending the five week wait and delivering a first payment within days rather than months," he added.
A DWP spokesperson said: "If people think they might be eligible for universal credit they should apply without delay and advance payments are available for those in urgent need.
"Last December we made payments of £774m to approximately 1.2 million households."