DAVID Cameron's government is poised to"blow the Union apart," Gordon Brown has warned, unless it makes urgent changes to the Scotland Bill.

The former prime minister accused the Conservatives of failing to devolve essential welfare powers to Holyrood, as agreed by the cross party Smith Commission which considered further powers for the Scottish parliament last year.

In a hard-hitting speech at Glasgow University, he said the failure to hand Holyrood clear powers to top up UK benefits would be seen as a betrayal of promises made before the independence referendum.

He told an audience of 300 academics and students: "Without the changes that give the welfare top-up powers to Scotland, we face a perfect storm - an explosive cocktail of measures that could blow the Union apart - the Conservative Government defying the Smith proposals on welfare, the very issue where their controversial imposition of cuts hits Scotland hard."

Mr Brown was the driving force behind what became known as "the vow," the pre-referendum promise to transfer extensive new tax and welfare powers to Holyrood.

David Cameron has said repeatedly the new Scotland Bill delivers powers agreed by the Smith Commission in full.

However, Mr Brown said the legislation as it stands failed to give Scottish ministers clear powers to top up UK benefits.

He also claimed UK minister could veto certain decisions taken at Holyrood, a complaint also made by the SNP but rejected by Scottish Secretary David Mundell.

He said the powers were essential to mitigate the impact of Conservative welfare cuts which, he warned, threatened to plunge a further 100,000 Scottish children into poverty.

The Bill faces its next Commons hurdle towards the end of this month.

Labour will attempt to amend it to beef up the welfare powers.

Mr Brown said:

"The Government should avoid what would be seen as a double betrayal - breaking their promises to the poor and breaking their promise to deliver the Smith recommendations in full on the very powers that are needed to counteract welfare cuts and the austerity they bring."

He added: "The Prime Minister was simply wrong to claim in the Commons in September that Smith has been delivered 'in full'.

"But with the two amendments we propose, we remove any suggestion the Scottish Parliament is denied the chance to top up welfare or is subject to any UK veto on implementing such a change."

Echoing comments he made during the referendum campaign, he said wanted to see "as near to a federal arrangement in the UK as you can be in a country where one nation is 85 per cent of the population"

He also said the Scotland Bill - if amended - would would consign the "old Britain" of centralised state and undivided parliamentary sovereignty to history.

"With these changes we propose, we can say the Smith Committee's recommendations on this are being met and the spirit of last September’s Vow is being upheld," he said.

Mr Brown, who stepped down as an MP in May, revealed he has continued to lobby ministers behind the scenes.

He released the text of a letter he wrote to David Cameron saying the changes were "essential".

He added: "The ball is now in the Conservative Government’s court. It is for them to agree to implement the Smith Commission in full, or end up being accused of failing to deliver the report's recommendation."Responding to the speech, a UK Government spokesman said: "The Scotland Bill delivers the Smith Commission Agreement in full with no vetos.

"The Scottish Parliament will be given the power to top up reserved benefits and no one is going to stand in their way."

Earlier in the day, David Mundell, the Secretary of State for Scotland, repeated his determination to have the Scotland Bill passed before May's Holyrood election.

He said the campaign should focus on how the parties would use the new tax and welfare powers, which are expected to come into force in April 2017.

He said: "It's very important that people in Scotland know ahead of next year's elections what the

The Scotland Bill will give MSPs control over income tax rates and bands, and will hand Holyrood half of the cash raised from VAT in Scotland while at the same time making it responsible for about £2.5 billion worth of welfare spending.

A UK Government spokesman made clear that the Scotland Bill would enable Holyrood to top-up its welfare powers.

He said: "The Scotland Bill delivers the Smith Commission Agreement in full with no vetoes. The Scottish Parliament will be given the power to top-up reserved benefits and no one is going to stand in their way."

David Mundell has already indicated that the legislation will be "substantively" amended before it is considered further by MPs later this month to change the language in certain areas to ensure that people are assured that the UK Government would not seek to veto any changes Holyrood desired to make.

The Scottish Secretary stressed how Whitehall wanted the Scottish Parliament to have the new tax powers available to it at the earliest possible date, April 2017, and that it was now up to the SNP Government to set out its proposals on tax and spend in the run-up to the May 2016 elections.

"Everyone in Scotland, even people who voted Yes, wants to see Scotland having tax and spending powers, why wouldn't you?" declared Mr Mundell. 

"Obviously, they want to get a fair settlement; we want to get a fair settlement and I'm absolutely sure that that can be achieved."

Confirming that he would be bringing forward "substantive amendments" to the Scotland Bill within the next two to three weeks, the Secretary of State added: "I want to make sure that anybody who is objective will sign off the Bill as meeting the Smith Commission in full; in word and spirit."