Image copyright PA Image caption The Scottish Parliament is assuming substantial new powers over tax and welfare

The future of the United Kingdom is being threatened by "piecemeal" devolution to England, Scotland, Wales and Northern Ireland, peers have said.

The cumulative impact that the transfer of powers was having on the "integrity" of the union had not been considered, the Lords Constitution Committee said.

Further granting of powers, it said, should be subject to an assessment of their effect on the UK's cohesion.

The SNP said the peers were "trundling out all the same old arguments".

The UK government has insisted the union is stronger as a result of the new settlement for Scotland, the proposed strengthening of the National Assembly for Wales and the Stormont Parliament in Northern Ireland and the Northern Powerhouse plan to empower Greater Manchester and other City regions by giving them responsibilities for health care, transport and planning.

'Taken for granted'

But the cross-party committee has expressed reservations about the pace of devolution and the thinking behind it, arguing it has been "ad hoc and reactive" with too little thought given to the preservation and enhancement of the union.

Lord Lang, the Conservative peer and former Scottish secretary who heads the committee, said the union was "being taken for granted" and any further devolution must be governed by a new set of "constitutional principles" that safeguarded its integrity.

Image caption Lord Lang said the union's health must be uppermost in politicians' minds

Under an agreement brokered in the aftermath of the No vote in the 2014 independence referendum, Holyrood is assuming substantial new powers over tax, including air passenger duty, borrowing and welfare as well as the right to legislate in a ream of new areas.

But the peers warned against Scotland taking any further steps towards "full fiscal autonomy", saying this would break the union apart. They also said the UK Parliament should scrutinise much more closely any future proposal for a second independence referendum.

While welcoming the more modest pace of devolution in England, the peers said the case for greater self-government for England remained "unresolved" and had clearly not been satisfied by either regional devolution deals or changes to voting procedures in the House of Commons giving MPs from English constituents a veto over legislation affecting England only.

The committee's main recommendations are:

Protecting UK-wide "core functions" underpinning the union in future devolution discussions

Publishing an impact assessment for the UK alongside any future devolution proposals

Scrapping the Barnett formula and replacing it with a "needs-based" funding model

Reforming the Joint Ministerial Committee for cross-UK working

Branding UK government services where they apply across the whole country

Rethinking the "imposition" of directly elected mayors on English regions which had voted against them

"Since 1999, devolution has been largely demand-led and piecemeal," Lord Lang said. "The committee saw no evidence of strategic thinking about its cumulative impact on the union as a whole.

"The government does not seem to recognise the pressures being placed on the United Kingdom by the ad hoc, reactive manner in which devolution has taken place, and continues to take place. It's now time to focus more on the union."

'Devolve and forget'

Successive UK governments, he added, had "failed to adapt" to devolution and the sheer number of policy areas which were no longer the preserve of Whitehall, a state of affairs which required much closer collaboration if UK-wide policies were to be implemented properly.

As such, the devolved administrations must be regarded as "established components" of the UK's unwritten constitution.

"Instead of the 'devolve and forget' attitude of the past, the UK government should be engaging with the devolved administrations across the whole breadth of government policy," he said.

"Not interfering, but co-operating and actively managing the cross-border and UK-wide implications of differing policy and service delivery choices. Shared and overlapping policy areas need to be handled sensibly, with each administration conscious of the interests of the others."

The committee is made up of pro-Unionist Conservative, Labour and Lib Dem peers as well crossbenchers. The SNP, Plaid Cymru and the Northern Irish political parties are not represented.

The SNP said the report was "irrelevant" and questioned the right of unelected peers to pass judgement on Scotland's governing arrangements.

"This report indicates a spectacular failure of the committee to grasp devolution, and its recommendations would break the vow and promises made to the people of Scotland during the referendum campaign," said SNP MP Pete Wishart.

"The House of Lords seem to be confused that in the Fiscal Framework - agreed between the Scottish and UK Governments - it is crystal clear that the Barnett formula will remain."