NICOLA Sturgeon is facing a row with unions after she claimed that some workers choose controversial agency contracts which make them pay to access their own wages.

The Scottish Trades Union Congress (STUC) said there are up to 75,000 workers employed under similar arrangements in Scotland.

Concerns have been raised over the practices after it emerged some workers on the Aberdeen bypass scheme – a major infrastructure project to create a road link between the north and south of the city – are working through agencies which charge them administration fees to access pay.

Sturgeon is due to address the STUC's annual congress on Monday in Aviemore, just weeks after she made the remark, during First Minister's Questions (FMQs), that some workers choose those arrangements.

On the eve of the STUC congress, a union official said Sturgeon should ban such practices.

Steven Dylan, Unite regional organiser for construction, said: "Nicola Sturgeon recently claimed that workers ‘choose’ to be on agency contracts.

"But if agency work is the only option for many construction workers, it is absurd to call this a choice.

"On the ground, our activity is exposing the ways the government’s choices are making work a misery.

"We have embarked on a major union organising initiative in the sector."

The row threatens to damage the close relationship between the SNP government and the STUC, which has been a key feature of the party’s 11 years in power. Alex Salmond set up a 'memorandum of understanding' with the STUC shortly after becoming first minister in 2007 to ensure the union would be consulted about policy on employment issues.

When asked by Scottish Labour leader Richard Leonard during FMQs what action she had taken over the issue at the Aberdeen bypass project, Sturgeon said: "It is at the discretion of individual employees if they choose to work through an agency."

A Scottish Government spokesman said Holyrood had limited powers to extend employment rights: “We strongly oppose the use of exploitative business practices and are using all powers at our disposal to promote fair work.

"We expect those who deliver public contracts to adopt ethical business practices, and have issued statutory guidance on how to consider fair work practices in procurement processes.

“Unfortunately employment law remains reserved to the UK Government.

"However, we will continue to press for the devolution of employment law so that we can put in place progressive policies based on the fair work principles which we share with the STUC.”