Glasgow council has come under pressure to give unions a place on the boards of its controversial spin-off firms.

The city's Labour administration pledged to hand seats at the top tables of its arm's-length external organisation or "Aleos" before the last municipal elections in 2012.

Now the opposition SNP, which has been sceptical about the accountability of the bodies for some years, has called for the promise to be delivered.

Big Aleos - such as construction and repair firm City Building and home services giant Cordia - essentially employ workers on the same terms as the council and are regarded as part of "the council family".

However, they are ruled by boards of councillors and executives that are formally separate from the local authority, which is the sole ultimate shareholder in the businesses.

The SNP's Graeme Hendry said: "It's been nearly three years since I heard Councillor Stephen Curran commit to worker places on Aleo boards, at a meeting between councillors and workers in the spring of 2012. Since then we have seen nothing change. Why the delay?

"The Scottish Government's Mather Review, which has cross-Party support, makes clear that such an approach is beneficial both for businesses and employees.

"With cross-Party support in the Council for this, I have written to the depute leader of the Council, Archie Graham, to ask for the Council to deliver this useful policy as soon as possible."

In his letter Mr Hendry stressed that workers seats on boards were standard practice in Germany. However, The Herald understands some Aleo executives are reluctant to see workers on their boards, citing concerns over confidentiality.

Trade unions have previously called for the abolition of aleos, particularly the loss-making City Parking, with their functions taken back in house.

Opposition councillors, including those critical of the Aleo system, do sit on the firms' boards.

Glasgow City Council was unable to respond to the SNP calls.