Peter Duncan has warned that a “proactive” approach is the only way to win the argument for the union.
The UK Government has authority over the constitution, but Boris Johnson has so far rejected calls to grant a repeat of the 2014 referendum, despite polls suggesting that a slim majority of Scots now back leaving the UK.
Polls also suggest that the SNP is on course for a resounding victory – and potential majority – at next year’s Holyrood election.
"The way for unionists to win the argument on independence is not to be seen to deny any clear mandate for a referendum that may exist," Mr Duncan told the Sunday Times.
"A 'no, never' approach will fan the flames for independence. The approach I'd recommend means there needs to be a plan for an early response after the election — dragging feet has never proven to be a good platform for winning any argument.
"The route to defending the Union is to be prepared, then proactive and positive. Carping negativity looks likely to end in disastrous failure."
But he warned Nationalists face a difficult task persuading Scots to vote for a union with Brussels, as part of the policy to rejoin the EU, instead of the historic union with the rest of Britain.
"That's a tough argument to make and a very difficult one to win," he added.
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