Mr Reid said this was because of its origins, dating back to an 832AD battle fought at Athelstaneford, in East Lothian, when the Picts under Angus mac Fergus defeated a larger force of Angles and Saxons.

MSPs were told the national flag has been devalued

"Fearful of the outcome, King Angus led prayers for deliverance and was rewarded by seeing a cloud formation of a white Saltire - the diagonal cross against which Saint Andrew had been martyred - against a clear blue sky," writes Mr Reid.

"The King vowed that if he gained the victory then Andrew would be the patron saint of Scotland.

"The Scots did win and the Saltire became the flag of Scotland.

"It is a matter of regret that this flag, with its proud origins, should be devalued by being made and flown with varying background shades. No other country's flag is made with varying backgrounds."

Mr Reid said in the letter that the Lord Lyon King of Arms, the authority on such matters, had declined to prescribe a precise shade of blue for the Saltire, a view accepted by Scottish ministers.

He called on the parliament to "remove variations which must be as puzzling to many outside Scotland as they are infuriating to many who live here".

He also urged MSPs to encourage the flying of the national flag, which he believed to be the world's oldest.