David Cameron has been accused of failing to "understand the Scottish mentality" after his high-profile intervention in the independence debate.
Canon Kenyon Wright, who was one of the founding fathers of devolution, hit out at the Prime Minister after his speech last week at the Olympic Park in east London.
In that address the Tory leader warned that the world would lose ''something very powerful and precious'' if the UK's ''family of nations'' broke up forever.
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The Prime Minister urged the people of England, Wales and Northern Ireland to send a message to Scotland as it prepares to vote on September 18: ''We want you to stay.''
But Canon Wright, who led the Scottish Constitutional Convention which campaigned for a devolved parliament in the 1980s and 1990s, hit out at the Prime Minister's intervention.
"Your actions so far lead me to the conviction that you barely understand the Scottish mentality," he said
In an open letter to the Prime Minister published in the Daily Record newspaper, Canon Wright said the campaign for the Union had seen "a procession of VIPs from England coming north to tell us that an independent Scotland would be impoverished, alone, divided and incompetent".
He added this had now been followed by a speech from the Prime Minister telling Scots "how much you love us".
But Canon Wright, who described himself as a Scot living in England, told Mr Cameron: "Neither threats not Valentines will work."
Instead the former constitutional campaigner argued the Union needed to be radically reformed if it is to be maintained.
He told the Prime Minister: "If you are serious about your deep longing to preserve the Union, there is something you can do. The only way to save the Union is to reform the Union."
He said that during the Westminster expenses scandal Mr Cameron had called for a "massive, sweeping, radical redistribution of power".
With the independence referendum looming, Canon Wright added: "Scotland is now giving you the chance to live up to those words."
He said that gave Mr Cameron the "chance to go down in history, not as the PM who said No, but as the statesman who said Yes to a new Union and a remodelled democracy".