Trains on one of Scotland's busiest railway lines are getting slower again just 12 months since a multi-million pound upgrade to increase their speed.

The number of weekday services between Glasgow and Edinburgh will be cut from ten to four from Dember 16.

Services which run the 42-minute route will thereafter be between one and three minutes slower, according to Scotland on Sunday.

ScotRail announced 42-minute journeys would soon become the norm for most off-peak trains, but now say the 'sheer volume' of trains on the line has made it impossible.

Last December, average journey times were cut from 51 to 47 minutes, but just two of more than 120 daily services between Scotland's major cities were 42 minutes.

READ MORE: Disabled campaigners hit out as fewer than half of Glasgow stations fully accessible

The number increased to ten in May - and was due to rise further.

In October 2018, the Scottish Government’s Transport Scotland agency said: “As we introduce even more class 385s to the ScotRail fleet, we can expect the May and December 2019 timetable changes to include further services with a 42-minute journey time.”

And soon after, ScotRail managing director Alex Hynes told MSPs the standard service pattern would be '42 minutes wherever possible' - other than at peak times and evenings.

Scottish Liberal Democrat transport spokesman Mike Rumbles said: “ScotRail promised passengers fast train trips between Edinburgh and Glasgow.

“Those rail users will now rightly be furious to find the pitiful number of trips being run to this ‘standard’ schedule is being further reduced.

“42-minute trips were supposed to be the standard service ‘wherever possible’.”

READ MORE: Average of almost 50 trains a day cancelled by ScotRail

A ScotRail spokesman said: “We try to deliver faster journeys in every timetable change – that’s on all routes, not just on the E&G.

“But this can’t be at the detriment of performance, which customers value highly.

“We’ll continue to deliver as many 42-minute journey times as possible with each timetable change, while ensuring our customer still receive a great service.”