The highly regarded pollster said the future of the union “will only be safe if people in Scotland come to believe in it”. His remarks come after a new poll indicated voters in Scotland are even more anti-Brexit now than they were in 2016.
A Panelbase survey found 72 percent of those north of the border now say they would vote Remain, compared to 62 percent who voted to keep in the single market in 2016.
Asked if they would vote to rejoin the Brussels club, 69 percent said they seek to reverse Brexit – up from 61 percent in January.
The poll of 1,133 adults in Scotland, carried out last week found there was also a heavy correlation between those who wanted to Remain and those who wanted independence from the UK.
It suggested 57 percent of those who voted Remain in 2016 would vote for Scottish independence, compared with 28 percent of Leave voters.
Poll result: Does Brexit justify a second Scottish Independence vote?
“But it is far from clear that the next prime minister will have the right strategy to achieve that.”
He added: “Both leadership candidates are committed to Brexit.
“But that simply makes their task on the Union more difficult.
“Scotland is even more opposed to Brexit than in 2016, when 62 percent voted against.”
The PanelBase poll suggested 51 percent of those in Scotland wanted to remain a part of the UK, while 49 percent wanted independence.
However, the survey suggested the thin support for the union is set to disappear when Ms Truss or Mr Sunak become Prime Minister.
If Mr Sunak wins the Tory leadership contest, 51 percent of Scots surveyed said they would support independence, while 49 percent would back the UK.
Under a Ms Truss premiership the margin was slightly bigger at 52 percent for independence compared to 48 perfect for the union.