The YouGov survey for the Sunday Times of 1,088 Scottish voters shows support for the party dropped from 50 per cent to 44 per cent in the Holyrood constituency vote and from 40 per cent to 36 per cent in the regional list, when compared to the results of the same poll in December.
Support for independence also dropped substantially, from 53 per cent to 47 per cent among decided voters. SNP support at Westminster dropped marginally from 43 per cent to 42 per cent.
First Minister Nicola Sturgeon has said she plans to run the next UK election as a “de-facto referendum” in the hopes of achieving a majority of votes north of the border and securing negotiations on independence.
But the First Minister’s approval rating has also suffered in recent months, dropping from a net of 7 per cent in October to -4 per cent.
The fieldwork for the poll was being done at the same time as a row over the imprisonment of double rapist Isla Bryson was raging in Scotland.
Bryson, who committed two rapes while she was a man, was initially housed in segregation at Cornton Vale near Stirling – Scotland’s only all-female prison – before being moved to the male estate after public and political outcry.
The saga led to Scottish justice secretary Keith Brown pausing the movement of transgender prisoners into women’s jails if they have a history of violence.
Professor John Curtice, of Strathclyde University, told the Sunday Times: “While the decline in support for independence is likely to be part of the explanation, it looks as though the battering the Scottish Government has suffered, especially on the issue of transgender prisoners, may also have taken its toll on SNP support.”
The party’s depute leader Keith Brown said: “While we take nothing for granted, this poll suggests that the SNP remains by far the most popular party in Scotland, and Nicola Sturgeon is by far the most popular leader.
“Scotland is an enormously wealthy, resource-rich country, but under Westminster control, we face being part of the worst performing economy in the developed world, with Brexit an unfolding a disaster that is making us all poorer.
“The fact that Keir Starmer’s Labour Party is now fully signed up to the Tory hard Brexit – and indeed backs their refusal to respect Scotland’s right to determine its own future – shows why independence for Scotland is now essential.
“Independence will mean a proper partnership of equals between Scotland and the rest of the UK instead of ever-tightening Westminster control.”