The Barnett Formula is "inadequate" and the UK Government should "reconsider" its usage, a House of Lords committee has recommended.

In a report into devolution's effect on the stability of the Union, the Lords call on the government to consider scrapping the Scottish Parliament's main funding formula.

The report states: "In our view, to perpetuate the use of the Barnett Formula, which takes no account of relative need, makes a mockery of the Government's duty to ensure a fair distribution of resources across the UK.

"We recommend that the UK Government reconsider its use of the inadequate Barnett Formula and establish a mechanism that takes into account the relative needs of different nations and regions in allocating funds."

The committee's report also states that the Lords believe that the Union is "under threat" from "piecemeal" style of devolution which has been adopted since 1999.

According to the report the UK Government has taken Scotland's place in the Union has been "for granted" and the Lords committee calls on ministers to "focus more on the Union".

Former Scottish secretary, and committee chair, Lord Lang says he "saw no evidence of strategic thinking" on the impact of further devolution across the period since 1999.

Lord Lang said: "We must stop taking the Union for granted. Since 1999, devolution has been largely demand-led and piecemeal. 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."

The committee recommends that government ministers develop a new mindset in regards to devolution.

The report states: "A new mindset is required at all levels of government - one that recognises the devolved institutions as now being established components of the UK's constitution.

"This new mindset will require abandoning a 'devolve and forget' attitude. Instead, the UK Government should engage with the devolved institutions across the whole breadth of government policy, co-operating and collaborating where possible. In particular, the Joint Ministerial Committee should be reformed to promote co-operation and collaboration, rather than grandstanding and gesture politics."

The Lords also stated that any second referendum on Scottish independence must be initiated through "primary legislation" from Westminster rather than the Scottish Parliament.

SNP MP Pete Wishart said the report is a "spectacular failure" to grasp devolution and would break 'the Vow' made before the independence referendum. Two days before the 2014 referendum the then leaders of all three major UK-wide parties committed to keeping the Barnett Formula in place.

Wishart said: "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.

"This House of Lords report is irrelevant and adds nothing to the debate. The decisions about the future of the government of Scotland should rightly be made by the people who live and work here.

"Decisions about Scotland's future should not be left to a group of cronies and politicians who have been rejected by the electorate and have no democratic accountability whatsoever.