Robin Barr, chairman of Irn-Bru maker AG Barr, is preparing to pass on the secret recipe of Scotland's favourite soft drink as he stands down after 31 years at the helm.

Barr will eventually pass on the recipe invented by his great-grandfather to his daughter Julie, who is the company secretary.

In the meantime, the 71-year-old will continue to go to the Irn-Bru factory in Cumbernauld, just outside Glasgow, once a month where he mixes the 32 ingredients in a sealed room.

Only one other unidentified person knows the recipe, which has been unchanged for 108 years. It is written down and stored in a bank vault at a secret location in Scotland. That person and Barr never travel on the same plane.

Barr's retirement means leadership of AG Barr passes out of the hands of the Barr family for the first time since the company was set up in 1875. Ronnie Hanna, a non-executive director, will step up to become chairman, as previously announced in March.

The company said in a statement that trading had been "excellent" in the first 15 weeks of the year, with sales "well ahead" of last year. Barr bought exotic fruit juice-brand Rubicon last year to expand its drinks portfolio.