Labour under fire over the "Vow Plus"

They said a new Labour government would give Holyrood full powers to create new benefits and top up UK entitlements such as the state pension and child benefit.

In a passionate speech to supporters, Mr Brown said the proposals went further than the package of measures recommended by the Smith Commission last October and insisted Holyrood would not be left "powerless" to tackle issues including youth unemployment and housing shortages.

But the surprise pledge prompted a scathing response from opponents.

The SNP dismissed it as a panic measure, while the Scottish Lib Dems, who backed the original pre-referendum "Vow" on more powers, said Labour were "making it up as they go along".

Under Labour's plans, which will be included in the party's election manifesto, a series of additional powers will be devolved.

Holyrood would gain the power to create new benefits and top up - but not reduce - UK payments.

Control over housing benefit and its £1.8billion budget would be transferred to Holyrood, in a move Mr Brown claimed would enable the Scottish Government to address the country's shortage of affordable homes.

The Scottish Parliament would also take control over incentives used to get the unemployed back to work, along with the UK Government's £500million Work Programme. Labour said the controversial scheme would be put in the hands of local councils.

Mr Brown, making his first foray into the general election campaign, said: "With the powers we are talking about today the Scottish Parliament will not be powerless to act on jobs, the housing crisis, youth unemployment and creating a more socially just Scotland.

"No longer will the SNP be able to practice this blame game. They would have to face up to their responsibility."

He added: "If people want independence, if they want to break away in all aspects from the rest of the Britain, to break away from UK pensions and passports, they will vote for the SNP.

"But if, as I believe, people want change, and the change they want is social justice and better policies for creating jobs, I believe people will see the real change to their lives will come not from the SNP but from electing a Labour government at Westminster under Ed Miliband."

Mr Murphy said Labour had no plans to use the powers to top up UK benefits in Scotland.

The former Prime Minister was a key architect of the "Vow," the pre-referendum pledge to devolve extensive new tax and welfare powers to Holyrood in the event of No vote.

A subsequent deal between the five Holyrood parties was turned into draft legislation last month.

At the time, it was hailed by Labour as giving Holyrood the power to create its own £2.5billion per year welfare system.

SNP deputy leader Stewart Hosie said: "Last month, Labour tried to tell people that the Vow had been delivered and there should be no more discussion of further powers for Scotland.

"Today, as more and more people back the SNP ahead of the coming election, Labour have been dragged along in Scotland's wake."

He said Labour's new proposals did not go "substantially further" than the original Smith Commission agreement.

He added: "Today's announcement from Jim Murphy and Gordon Brown shows that their disastrous poll ratings are causing panic in Labour's ranks."

Scots Lib Dem leader Willie Rennie said: "Labour seem to be making it up as they go along.

"In the Smith Commission they were the most cautious on transferring welfare powers to Scottish Parliament.

"Within weeks of agreeing this radical package they now claim it's not enough."

The package of employment and welfare powers agreed by the parties was criticised by unions and charities.

The STUC welcomed the new pledge.

General Secretary Grahame Smith said there was "absolutely no reason" why the additonal powers should not be used to top up benefits in Scotland.

He said: "This development confirms our view that there is a continuing and very live debate over which powers should be devolved and how that should be implemented."