SUPPORT for the SNP has increased despite opposition attacks over the Scottish Government's handling of the Forth Road Bridge closure and the NHS, a new poll has revealed.

The TNS survey found that the nationalists have extended their lead over Labour ahead of next May's Holyrood election, with Kezia Dugdale's party seeing its ratings drop since last month in both constituency and list voting intentions.

It was also found that more people in Scotland oppose the renewal of the Trident nuclear deterrent than support it, although there is not an overall majority in favour of scrapping the weapons system among the public north of the border. New evidence also emerged that interest in the upcoming election is waning, less than six months out from the vote.

Overall, the SNP is on course to take 58 per cent of the constituency vote next May, unchanged on last month, while it is on course to take 54 per cent of the list vote, a rise of two per cent, according to the poll.

Labour is polling at 21 per cent in the constituency section, a drop of three points, while it is predicted to take just a fifth of regional list votes, a drop of five per cent.

The survey of 1,035 over-16s, carried out between November 16 and December 14, found Conservative and Liberal Democrat support held steady at 12 per cent and 4 per cent respectively in the constituency section with others up two points at 4 per cent.

On the regional list, the Conservatives are on 12 per cent, a rise of one, the Liberal Democrats fell one point to four and there was good news for the Greens, who were on nine per cent, an increase of four points.

According to the ScotlandVotes seat calculator, if the results are repeated next May the SNP would win a clear majority with 78 seats. Labour would drop to 25, and rely solely on list seats allocated through a system of proportional representation, after losing every constituency. However, they would remain the main opposition.

The Tories would boast 15 seats, the LibDems two while the Greens would surge from two to nine, becoming the fourth-largest party in the Scottish Parliament.

Tom Costley, the head of TNS Scotland, said: "The past month has seen the political agenda return to devolved issues such as healthcare and transport, with opposition politicians attacking the SNP government’s record on hospital provision and on maintenance of the Forth Bridge. The criticism appears to have had little or no effect on support for the SNP.

"One interesting feature of the poll is that the number of those who say they are certain to vote in 2016 has been declining, and now stands at 58 per cent, down from 64 per cent as recently as the TNS poll in September. The turnout in Scotland in the May 7 general election was 71 per cent.

"It may be that, faced with the SNP’s huge lead in the polls, a number of voters feel that their vote would not influence the result. However, it seems unlikely that turnout in May will be as low as the 50 per cent recorded in the 2011 Holyrood elections."

Asked about issues that would be important in deciding how they vote, healthcare topped the list with 51 per cent, though this was eight points lower than when TNS last asked the question in June. The economy, which was the fifth most important concern in June at 30 per cent, rose to joint second place – with education and training – on 40 per cent.

Employment and job prospects were cited as important by 39 per cent of respondents and pensions/benefits by 37 per cent.

TNS also asked respondents their opinion on replacing Trident with a new nuclear weapons system. More than a third opposed the proposal - 38 per cent - with 29 per cent supporting it and more than a quarter undecided.