WESTMINSTER plans to shake up the benefits system before related powers are passed to Holyrood could "wreak havoc" with the most vulnerable people in society, campaigners warn today.

Rolling out Universal Credit (UC) in Scotland next month would be risky and premature as it would cut directly across future welfare changes recommended by the Smith Commission on devolution, they say.

A cross-section of civic society including housing associations, anti-poverty groups and the ­moderator of the Church of ­Scotland today make their concerns public in an open letter to Conservative Work and Pensions ­Secretary Iain Duncan Smith, imploring him to suspend the roll-out of UC "immediately" or risk chaos further down the line.

The council umbrella group Cosla is also backing a suspension of next month's roll-out.

A key part of Coalition welfare reforms but dogged by problems, UC merges Jobseeker's Allowance, Housing Benefit, Income Support, Working Tax Credit, Child Tax Credit, and Employment and Support Allowance into a single new payment. Although meant as an ­improvement, it has been blamed for more claimants going into arrears, as payments are only made once a month and rents no longer go directly to social landlords - and many vulnerable tenants simply spend the money.

A large-scale roll-out is due to take place across the UK between February and April, affecting new single claimants in nine ­Scottish council areas.

Under the plan agreed by the Smith Commission, UC will remain a reserved benefit, run and delivered by the Department for Work and Pensions (DWP). However, Scottish ministers will get powers to vary aspects of UC, including payment timings and whether to pay housing costs directly to landlords.

Holyrood will also to get the power to vary the housing elements of UC, including the so-called bedroom tax, housing allowance rates and some deductions.

The open letter to Iain Duncan Smith warns that if the roll-out goes ahead before the new legislation takes shape a clash is inevitable, and the system will have to be overhauled within a few years.

It says: "Driving through UC in Scotland at this stage will create unnecessary administrative complication in an already complex process. The only sensible way to roll out UC in Scotland is to do it once, when the Scotland-specific elements have been ­carefully planned and incorporated into it.

"This will avoid wasting precious time and scarce resources, and will protect vulnerable people in our society from unnecessary bureaucratic change that could wreak havoc."

The 57 signatories, which include 12 housing associations that could lose money from increased tenant arrears, stress the suspension call is "not about politics" but protecting the vulnerable and "responsible, effective governance".

Other signatories include the Moderator of the Church of ­Scotland the Rt Rev John Chalmers, the Trussell Trust, STUC, ­Scottish Women's Aid, Money Advice Scotland, the Equality Network, Inclusion Scotland, SCVO, the Scottish Association of Social Work, Engender, and the Common Weal think tank.

The letter was organised by Mary Taylor, chief executive of the Scottish Federation of Ho­using Associations and a member of the Scotland Office stakeholder group on the draft Smith legislation.

She said the roll-out should be paused "at least until autumn", until the new laws became clearer.

"I'm not convinced that rolling something out that is both not ready and subject to further change is a wise move," she said.

"[UC] may ultimately be a good thing, but it will only work if it's introduced with smooth arrangements and a proper system."

She said a botched roll-out would hit the most vulnerable hardest.

"People on UC are typically going to be right up against the wall and having a difficult time.

"My main concern is for the individuals concerned."

The DWP said constant ­improvements were being made to UC in light of pilots in Inverness and elsewhere, and the government would continue to work closely with landlords.

A spokesman said: "UC is ­transforming lives. Already claimants in Inverness are moving into work faster and staying in work longer. By rolling out to one in three of Scotland's Jobcentres by spring, more people will benefit from a new system that makes work pay.

"To halt this progress now would be to disadvantage jobseekers across Scotland."