The Scottish Government is responsible for overcrowding on the country's trains, according to ScotRail's operator.

Abellio, which took over the franchise in 2015, hit out at plans put forward by the transport minister Humza Yousaf to place a rival public-sector bid when the franchise's contract comes up for renegotiation in 2020.

Yousaf has criticised Abellio's performance in recent weeks and threatened the company with "very, very serious consequences for ScotRail, for Abellio, if they do not improve their performance."

Recent figures show the firm's punctuality performance is below the target of 91.3% it is expected to reach.

Abellio insists some problems on the network, such as overcrowding, are the fault of the Scottish Government.

Charlotte Twyning, director of policy, strategy and communications for Abellio UK, said: "There is nothing new in discussions about a public-sector bid for Scotland's railways and we have no problem competing with public bids.

"However, it should be recognised that half of the rail industry is already nationalised in the form of Network Rail and any operator that runs the ScotRail franchise - public or private - does so to a tightly specified Scottish Government contract, which means that ministers set fares and essentially determine how many seats are available and therefore how much overcrowding exists across the network.

Twyning continued: "We acknowledge that the performance of the ScotRail Alliance is not good enough but we are working hard with our clients - Transport Scotland and the Scottish Government - to make the changes necessary to improve the service to rail passengers.

"We are also encouraging Network Rail in particular to raise its game.

"However, rail passengers are better served by organisations and individuals collaborating in customers' interests and not their own, and we encourage all parties to do so."

On Thursday, the First Minister apologised to commuters who faced hours of delays after a train broke down between Haymarket and Waverley causing the cancellation of rush hour morning services between Glasgow and Edinburgh.

The Scottish Government said Abellio put forward plans to tackle overcrowding when put in its bid for the franchise and it "must" deliver on them.

A government spokesman said:"In terms of overcrowding, it was for each bidder for the re-franchise to show how they would project growth and the right trains to minimise crowding.

"Abellio did that with a plan that showed better utilisation of the existing fleet and more trains coming to meet growing demand. In addition, we are currently taking forward the largest ever train improvement programme with £475m being invested in bringing more seats, better reliability, availability and upgrading and refurbishing the existing fleet.

"We always have and will continue to look for every opportunity to add to the train fleet to meet demand and will work with Abellio to identify and take opportunities. In the meantime, under the contract, it is absolutely for Abellio to manage the fleet and other resources to minimise overcrowding, this is an important aspect of the perfomance improvement plan and one they must deliver on."

Yousaf will deliver a parliamentary statement to MSPs later this week.