Barclay made his Scotland debut at his first of three Rugby World Cups, in 2007, against New Zealand at BT Murrayfield Stadium, one day before his 21st birthday. He would go on to make 76 appearances for his country.

Announcing his decision on social media, he said: ” All good things must come to an end.

“After much thought I have decided the time is right to step down from the international game. This isn’t an easy decision to make; playing for Scotland is the culmination of years of hard work and dedication and undoubtedly one the best things I will achieve in my life. But for me and my family it feels like the right time.

“It has truly been a privilege to wear the jersey. Not many people can say they got to live their boyhood dream, but I was lucky enough to have that honour and it is something I am unbelievably proud of.

“To my family and friends for supporting me all over the world, a big thank you. Most importantly though, thank you to my wife for her support looking after our growing family when I have been away and for dealing with the inevitable ups and downs associated with playing international rugby.

“Not the end of my rugby journey, but the end of one chapter. Thanks for the memories.”

The first recipient of the prestigious Macphail Scholarship, Barclay’s promise was evident from his early teens and – less than a month after his 18th birthday – he was included in the extended squad for Scotland’s Autumn Tests, before he had made his Glasgow debut, which followed in November that year.

In February 2007 he made a substitute appearance for Scotland A who beat their Italian counterparts and then appeared in all three of Scotland A’s games in the 2007 Churchill Cup in England, making his first start in the 13-9 win against the USA at Henley.

That was followed by his selection as one of five uncapped players in Scotland’s squad for World Cup preparation and his debut against the All Blacks.

Even in defeat in that match he produced clear indications as open-side flanker that a long international career lay ahead for him.

His Six Nations debut followed in the opening match of the 2008 tournament against France, followed by facing Wales in Cardiff and the second Test win against Argentina in Buenos Aires, scoring his first Scotland try against Canada in the autumn that year.

His second came against Wales in Scotland’s second Six Nations match of 2010 before being named man of the match for another all-action display as Scotland defeated Argentina 13-9 in Mar del Plata to clinch a historic 2-0 series victory in the summer of 2010 – the first time Scotland had achieved a series success in 50 years of touring.

In the first Autumn Test that year – against New Zealand – he took on the role of acting Scotland captain when skipper and scrum-half Mike Blair was injured. He was then named man of the match for his contribution to Scotland’s home victory against South Africa.

John made a further 20 Test appearances in the two years that followed, starting all but four matches, including a start in all of Scotland’s matches in the 2011 Six Nations, and three RWC2011 matches in the New Zealand hosted showpiece.

The following year would see him feature heavily once again, as a mainstay in the Six Nations, Summer Tour and Autumn Tests, with his part in a historic victory over Australia in a rain-soaked Newcastle, New South Wales a clear highlight.

A year passed before John was again seen in a Scotland jersey, appearing twice in the Autumn Tests that year before a two-year spell of absence, which ended with a try-scoring replacement appearance in the 2015 Summer Test against Italy, as part of the extended RWC2015 squad.

Despite missing out on selection for the tournament, John was determined to cement his return, with his commitment to club form rewarded with selection in 2016 Six Nations squad; a chance he took, and was subsequently rewarded, with a start in all five championship games – scoring his fourth international try against Italy, and making a 50th Test appearance in the final game, against Ireland.

Barclay remained in Scotland contention for the following season but came into his own in the 2017 Six Nations when he took on the captaincy from scrum-half Greig Laidlaw, who was injured mid-championship.

He remained the Scotland captain for the 2018 tournament and featured in every Test of the campaign, leading Scotland to triumph over England for the first time in 10 years, as well as wins over France and Australia in Sydney, meaning the Calcutta Cup had the Auld Alliance Trophy and Hopetoun Cup for company that year.

Successfully earning selection in the final 31-man group for Rugby World Cup 2019 was the next target and landmark, where – it would later transpire – he would play his last match as a try-scoring captain in the 61-0 defeat of Russia in Shizuoka.

Scotland Head Coach Gregor Townsend said: “I was fortunate enough to coach John at both club and national level, and throughout that time he was a very consistent and committed performer.

“He put his body on the line for his country on many occasions and was always very proud and passionate about playing for Scotland. That, combined with his rugby intelligence, are key qualities that have made him the player he is today.

“He’s had some major moments and performances that have helped Scotland win – the win against Australia in Sydney when he led the team so well, and our win against England in the Calcutta cup in 2018 spring to mind from recent times – where his presence around the breakdown, his work-rate and support play in attack showed him leading by example and really drove the team to success.

“He’s been a pleasure to coach over the last few years and we wish him well him in his future career. He still has a lot to give as player but he can be very proud of what he’s achieved for Scotland.”