"Margaret Greenwood, the shadow minister for employment, said: “Universal credit is causing misery and hardship for thousands of families this Christmas and councils are being expected to pick up the pieces. This is yet more evidence that the government should immediately pause the roll out of universal credit so its fundamental flaws can be fixed.”
Newcastle city council reported that it was spending £390,000 supporting UC claimants, almost a quarter of which was for additional rent arrears support.
Liverpool city council said it had spent £175,000 from its local welfare provision scheme on UC claimants, while Shropshire council said it had set aside £20,000 to help food banks to “diversify the type of help they are able to give specifically to suit universal credit.”
In London, Tower Hamlets council said it had set aside £5m over three years to help those affected by the shift to UC, while Barking and Dagenham is budgeting £50,000 from January 2018.
In total, 26 councils said they had set aside extra resources or anticipated increased demand for welfare support as the UC rollout reaches their area."
From the article I linked to.
This is extra money on top of the discretionary housing payments which are provided by the government.
I can find no mention of Shelter saying that councils are holding back money.