He claimed the abolitionists supporting leadership contender Murdo Fraser’s proposal to ditch the party and replace it with a new centre-right movement was misguided and threatened to plunge the party into years of internal strife.
His comments came as one of Scotland’s top businessmen, Ben Thomson, endorsed Mr Fraser, saying his proposals made sense for Conservatives and Scottish politics.
Lord Forsyth, who is backing Ruth Davidson, the other frontrunner in the leadership contest, said: “Harping back to the halcyon days of the 1950s when Scottish Conservatives had 50% of the vote is misleading and misguided.
“There were only two candidates in most constituencies, there were no credible SNP candidates and the mere handful of Liberal Unionists supported the Tories.
“Abandoning our party now would be the greatest political error since Bonnie Prince Charlie, on the advice of fainthearts, turned back at Derby to face certain defeat.”
In a letter, Mr Thomson, a merchant banker and former chief executive and chairman of the Noble Group, said losing the Conservative brand and direct association with the party at Westminster would help create a break from the past and galvanise its current MSPs into creating policies suited to Scotland.
Mr Thomson, who has not put any funding into Mr Fraser’s campaign, is also chairman of non-party political think-tank Reform Scotland and a spokesman stressed the letter expressed his personal views, not the organisation’s.
After hearing Lord Forsyth’s comments, Mr Fraser’s campaign manager, Liz Smith, warned it was “important that we avoid negative campaigning and temper our language”.
She added: “It is clear that our party has an identity problem which cannot be solved by a new leader alone or by new policies. We respect the party’s establishment figures, and we respect the view expressed by some of them that we do not need to change. However, we fundamentally disagree.”