ScotRail have come under fire after admitting that it will not hit a target of stopping the dumping of human excrement onto railway tracks more than two years after the practice was meant to be scrapped.

Unions have been appalled that high-speed trains being brought into service in Scotland will continue to dump human waste onto train tracks into 2020.

A deal between unions and the Scottish government was meant to scrap practice by December 2017 but a shake-up in the ScotRail fleet was expected to lead to its return.

Now it has emerged that delays to a fleet of refurbished trains means ScotRail will still be using rolling stock without toilet waste tanks next year even though further discussions meant it was supposed to end by 2020.

READ MORE: Anger as trains to dump human excrement on Scotland's tracks again

ScotRail was meant to have received 26 refurbished high-speed trains, with waste tanks fitted, for routes linking Scotland's seven cities from rail firm Wabtec by December last year. It is understood only nine have been delivered so far.

HeraldScotland: ScotRail train stock

ScotRail has been in negotiations to accelerate the refurbishment programme. The train operator says 17 of the carriages are now being refurbished in Kilmarnock.

With newly-refurbished trains being delayed some older trains are being used as an interim measure to cover services which do not have tanks to store the waste to make up for the shortfall and they will still be in operation next year.

Similar issues with delays to refurbished trains emerged in October, last year.

Then, the RMT general secretary, Mick Cash, wrote to the ScotRail's managing director, Alex Hynes, and the Scottish government transport secretary, Michael Matheson, calling for urgent action on the issue and highlighting the “serious health risks” posed.

Then, a ScotRail spokesman acknowledged that it was likely to be some time before newly refurbished trains with human waste tanks would be ready.

Now ScotRail say: “We’re working with suppliers to ensure the refurbishment of our fleet of high-speed InterCity trains is completed as soon as possible.”

READ MORE: Sturgeon - Trains dumping human excrement is 'not a practice we support'

The transport staff union, TSSA has condemned ScotRail for failing to end the dumping of human waste on the tracks by 2020.

Manuel Cortes, TSSA General Secretary said, “More than a year ago ScotRail assured us, hand on heart, that they were working flat out to get their high speed trains equipped with retention tanks as soon as possible. Yet trains are still waiting for the tanks to be fitted.

“This simply isn’t good enough. In the 21st century passengers shouldn’t still have to be warned not to flush the toilet while the train is standing in the station. My heart goes out to the workers on the lines facing the unsavoury prospect of finding human waste on tracks.

“What we’re seeing is the long-term effects of Abellio’s penny-wise and pound-foolish management of ScotRail. They grabbed a quick buck by laying off engineering staff in the first years of the franchise leaving the department permanently understaffed.

"Then they bought 40-year-old museum pieces from Great Western rather than invest in new trains. And they rounded it off by getting the cheapest refurbishment contract they could – with Wabtec in Doncaster rather than investing in Scottish craftsmanship at the Caley [the historic St Rollox rail depot in Springburn which has shut down].

"And they’re surprised that Wabtec’s work hasn’t been up to scratch, the trains aren’t fit for purpose and their engineering department hasn’t enough staff to cope. But you can’t run a 21st century railway with pound-shop parts.

“But the real blame here lies with the privatisation system itself. If the railways were held in public ownership, run for the good of Scotland, as Labour have promised in their manifesto, then we wouldn’t see the relentless drive for profits above all else that leaves human waste flushed onto Scottish railway lines.”

In November, last year, Nicola Sturgeon insisted trains dumping human excrement onto tracks is “not a practice we support”.