NICOLA Sturgeon has insisted trains dumping human excrement onto tracks is “not a practice we support”.

The First Minister spoke out after ScotRail said it was temporarily bringing back older trains without waste tanks attached.

The firm described the models as “classic”, but unions raised fears rail workers will be sprayed with human waste.

Scottish Liberal Democrat transport spokesperson Mike Rumbles said ScotRail’s performance “stinks”, adding: “And now that has been taken to an all too literal level.”

Speaking during First Minister’s Questions, Ms Sturgeon said: “It’s not a practice that we support, and ScotRail have also said that it’s not one they want to see continue.

“It’s an interim measure. It is regrettable, and ScotRail are working to mitigate the issue as soon as possible.

“The Scottish Government, of course, has directly funded previous installation programmes to eradicate that practice across ScotRail’s fleet.

“It will be necessary to introduce some un-refurbished high-speed trains into service for an interim period, but it is important that ScotRail works to resolve that as quickly as possible.”

Trains stopped dumping waste in December 2017 following a deal between the unions and the Scottish Government.

But the reintroduction of older trains dating from the 1970s will see the practice return.

ScotRail said the models will only be used until the arrival of a new, refurbished fleet.

It previously said: "We are working flat out to get more upgraded trains into service as quickly as possible, but our supplier, Wabtec, is behind schedule and has let us down.

"To ensure we can deliver services for our customers we will introduce classic high-speed trains until enough upgraded Inter7City trains are available.

"These classic trains will not have retention tanks, which is far from ideal. We are doing everything we can to mitigate this as soon as possible."

Mr Rumbles said: "Carriages are jam-packed because the new fleets are well behind schedule and now we are getting 1970s trains that are dumping waste.

"This practice is unacceptable in a 21st century rail system. I'm disappointed that the First Minister was unable to say how long this practice will continue or what measures will be put in place to reduce the disgusting impact of this on railway workers and those who live close to lines.

"The whiff of incompetence is strong here."