Glasgow's largest bus operator is calling for action to tackle congestion in the city as a report found an "alarming" decline in passenger numbers as journey times increase.

Transport group Greener Journeys said bus speeds are falling faster in Glasgow than anywhere else in the UK, putting the future of the sector "under threat".

Bus speeds in the city are dropping by 15% per decade compared to an average of 10% for the rest of the UK, leading to a 22% decline in passenger numbers across the Strathclyde area in the last decade.

The report also claimed that if journey times continue to decline at the current rate, bus passenger numbers will drop by between 10% and 14% every 10 years.

Report author Professor David Begg said that traffic congestion in Glasgow was a key factor behind the fall in bus speeds and passenger numbers.

"If you combine falling bus speeds with relatively cheap subsidised competition from an impressive urban rail network and cheap and abundant public car parking, it explains why Glasgow has experienced an alarming decline in bus patronage over the past decade," Prof Begg said.

"Traffic congestion is a disease which, left unchecked, will destroy the bus sector.

"This is a dire and sensational prediction, but the evidence uncovered in this research leads to no other conclusion.

"Urgent action is required from industry, local government and Whitehall to reduce people's reliance on cars and encourage more sustainable modes of transport."

First Glasgow said congestion leads to increased operating costs which, in turn, leads to increased fares further deterring passengers.

Glasgow City Council introduced a bus gate around George Square and Nelson Mandela Place in 2014 to reduce traffic during the day.

First Glasgow interim managing director Alex Perry said: "This report paints a stark picture for bus transport in Glasgow but we believe that by working with our partners and stakeholders in the city to reduce journey times we can turn things around.

"Sadly, significant falls in bus patronage are often portrayed as being primarily due to the actions of the bus operators.

"However, this report lays bare the significant impact congestion has on our city and on bus passengers.

"By working in partnership to make space for buses we can deliver sustained benefits for passengers and the local economy."