Nicola Sturgeon has insisted the SNP are the "real opposition" to the Tories at Westminster after Labour MPs abstained in a key Commons vote on welfare reforms.

The Scottish First Minister said Labour's decision not to oppose the "ideological assault" on low-paid workers showed the party had "lost its sense of purpose".

While SNP MPs voted against the Welfare Reform and Work Bill, the majority of Labour MPs abstained - although 48 rebels did defy interim leader Harriet Harman and vote against it.

Ms Sturgeon, the SNP leader, said: "If Labour had voted with the SNP against George Osborne's assault on those on low incomes, then the Tories could have been beaten last night.

"Instead Labour sat on their hands. If Labour is not about standing up for the vulnerable, trying to lift people out of poverty and help those who are working hard to make ends meet, then what on earth is Labour for?"

Ms Sturgeon said she was "sadly not" surprised by the party's decision to abstain, adding: "Labour seem to have lost any sense of purpose or any sense of direction.

"It really does beg a fundamental question, if Labour is not about opposing a Tory government that is waging an ideological assault not on skivers who don't want to work, but on people who are working hard on low incomes, if Labour is not about opposing that, what is Labour for?

"Last night just proves that Labour has lost any sense of purpose and it will be the SNP who increasingly will form the real opposition in the House of Commons."

Ms Sturgeon made the comments as she campaigned in Glasgow, where four council by-elections are due to be held on August 6.

"People across Glasgow will be looking in disgust at how Labour voted on the welfare changes in the House of Commons last night," the First Minister said.

In May's general election, voters in Scotland's largest city returned seven SNP MPs while opinion polls continue to give the nationalists a strong lead over their rivals.

Despite that, Ms Sturgeon said she is "taking nothing for granted" in the by-elections, adding: "One of Labour's many problems is that it has taken voters for granted and assumed people would always vote Labour. The SNP will never do that."

Ms Sturgeon continued: "In Glasgow there is a strong sense that the city council is not as accountable to people across the city as it should be and if we can elect four new SNP councillors we can make sure there is a strong opposition in the city chambers standing up for people across the city."

Hannah Bardell MP, the SNP spokeswoman for fair work and employment, said Labour's decision to abstain in the welfare vote in the House of Commons was "unforgivable".

She said: "The welfare cuts that the Tories are imposing need to be opposed for the sake of the millions of families and vulnerable people across the UK who stand to be made much worse off.

"The clear evidence of Monday night is that the SNP are leading that opposition while Labour sat on their hands and abstained, which was unforgivable."

Ms Bardell, the MP for Livingston, said: "MPs from every opposition party - including the Northern Irish parties - voted against the welfare cuts bill, and Labour could and should have united to work with a united opposition to vote against it.

"Given that the Tories only mustered 308 votes for this miserable and hard-hearted Bill, a united opposition had the chance to defeat it - but Labour failed even to try. That will haunt Labour through to next year's Scottish Parliament election and far beyond."