Labour leader Jeremy Corbyn has been accused of a "pathetic" lack of leadership after rejecting a call to join a cross-party coalition to keep Britain in the EU single market after Brexit.
The SNP said the Mr Corbyn was failing millions of working families after he turned down an invitation to co-ordinate efforts with other parties to block Government moves to take the UK out of the single market when it leaves the EU.
Ian Blackford, the SNP leader at Westminster, is due to meet with the leaders of the Liberal Democrats, Plaid Cymru and the Green Party on Monday to discuss a common approach to the issue in the weeks ahead.
But in a letter to Mr Blackford, Mr Corbyn declined an offer to take part, saying the initiative was based on a "flawed" assumption about the nature of the single market.
Labour has repeatedly said it wants to secure a Brexit deal which retains the benefits of the single market and the customs union, but has stopped short of saying it would remain in the single market beyond a limited transition period after the UK leaves the EU in March 2019.
In his letter to Mr Blackford, Mr Corbyn said the summit "appears to be based on the flawed assumption that the single market is a membership club".
"It is not," Mr Corbyn wrote.
"We have consistently said that we are committed to negotiating to keep the benefits of the single market and securing the best possible deal for the whole of Britain, which protects our economy and the people of this country, whilst at the same time respecting the result of the referendum."
However Mr Blackford warned that unless opposition parties were able to prevent the Government leaving the single market and the customs union, it would take a "wrecking ball" to the economy.
"With just over a year to go before the UK is set to leave the EU, it is now more important than ever that we have a united and effective opposition - holding the UK government to account, and doing everything within our power to prevent this Tory economic catastrophe," he said.
"It is unbelievable - and frankly, utterly pathetic - that on the single biggest issue facing the country Jeremy Corbyn has failed to show any leadership whatsoever, and is now rejecting this crucial chance to build a cross-party coalition in the national interest.
"His absence from this meeting is deeply disappointing but it is not surprising given his absence from the EU referendum campaign and the national debate on Brexit ever since."
A Labour party spokesman dismissed the cross-party meeting as "little more than a political gimmick".
"We will work with all parties to hold this Government to account every step of the way and achieve a jobs-first Brexit that puts living standards and the economy first," the spokesman said.