KEZIA Dugdale’s proposals to transform the UK into a federal system “should be taken with a pinch of salt” according to the SNP MSP who sat with Labour representatives on the body which examined extending Holyrood’s powers following the independence referendum.

Linda Fabiani said Dugdale’s party had spoken often about giving the Scottish Parliament more responsibilities but had failed to act, and described the plan as its “default” response to falling support.

“Labour have been promising a supercharged, powerhouse, federalism-max for years – and consistently failing to deliver it. In fact, it was Labour politicians that specifically blocked the powers over the minimum wage they are now asking for. This is just the latest version of the same old song,” she said.

“Kezia Dugdale is always quick to accuse others of obsessing over the constitution, but Scottish Labour’s default answer to bad polling numbers is to promise powers that they don’t deliver. Billions of years from now I half expect Labour politicians to be staring into the dying sun calling for a constitutional convention.”

She added: “Voters would be forgiven for taking this latest intervention with a pinch of salt.”

In her speech in London, Dugdale set out a plan for a new Act of Union, claiming transforming the UK into a federal system would help “heal our divided society”.

She said the new Act would mean increased remits for Holyrood, Cardiff Bay, Stormont and the English regions with the new powers including some of the responsibilities such as fisheries, agriculture and workers’ rights potentially coming to Scotland once they had been repatriated from Brussels.

During a radio interview she also reversed her position on Scotland’s Brexit deal.

In July, she said Scotland could “retain its place” in the EU through a federalist solution.

But yesterday, she ruled that out, telling Radio 4’s Today programme: “It’s the United Kingdom that’s a member of the European Union and what we need is the best possible deal for the whole of the UK.”

Dugdale has written to Prime Minister Theresa May to call for a “people’s constitutional convention” to decide on a “new political settlement” for the whole country and said Labour could go ahead with that plan even without the support of the government.

“After more than 300 years, it is time for a new Act of Union to safeguard our family of nations for generations to come,” she said in her speech at the IPPR think tank.

“This would mean a radical reshaping of our country along federal lines where every component part of the United Kingdom – Scotland, Wales, Northern Ireland and the English regions – take more responsibility for what happens in their own communities, but where we still maintain the protection of being part of a greater whole as the UK.”

Dugdale said her plan, which she will ask her party members to back at their conference in February, would help address concerns about Scotland’s future after Brexit.

“I believe a new federal UK, with a strong Scotland able to carry forward a significant relationship with Europe, is what people across Scotland have voted for.

“It goes far to meet the dual mandates of the Scottish people and gives us the best protection for our jobs and our economy.”

She added: “Our union must be saved. We must heal our divided society. It is only Labour that can do that.”

Dugdale was not holding talks with Labour leader Jeremy Corbyn even though her speech was taking place less than a mile away from the Houses of Parliament.

Patrick Harvie, the Scottish Greens’ co-convener, said the proposals would not keep Scotland in the EU despite the 62 per cent vote north of the Border to Remain in the bloc and he could not envisage the new powers being delivered by the UK Government.

“It’s unfortunate that while Greens and others are working collaboratively and imaginatively to come up with ways to keep Scotland in the European Union, Scottish Labour chooses this valuable time to discuss changes in the UK constitution which seem vanishingly unlikely to happen, instead of the very real and urgent threat posed by Brexit,” he said.

“None of what Labour proposes will keep Scotland in Europe, nor would it prevent the democratic wishes of the people of Scotland from being ignored again. It’s also unclear what evidence Kezia Dugdale has for support in the rest of the UK for this change. At present, the UK Government appears unwilling to consider far more modest proposals.”

She also came under attack from the Scottish Conservatives who accused of her stoking “more constitutional uncertainty” with calls for a new Act of Union.