PRESSURE was mounting last night for a "devo-max" option to be part of the Scottish independence referendum, as the SNP Government was again warned not to attempt to hold its own poll.

A new survey – taken last week as the row raged between the SNP Government and the Westminster Coalition over the legality of a referendum – showed the Nationalists are picking up support for their ambition to split from the rest of the UK.

While 61% of those questioned for the YouGov survey oppose independence, 39% were in favour. More than half (58%) want to give Holyrood power over all its finances against 42% who don't. When asked if – regardless of how they would vote – there should be a straight yes-no on the ballot papers to the independence question, 43% agreed. But 46% said there should be a second question – the "devo-max" option – about giving Holyrood more powers.

The survey was taken after Prime Minister David Cameron reignited the debate by demanding Alex Salmond come clean about the timing of the poll, which sparked a bitter row between the two governments. Mr Salmond later said it would be held in the autumn of 2014.

Canon Kenyon Wright, one of the leading architects of devolution, later told The Herald not having a second question would "disenfranchise" many Scots.

Last night, Angus Robertson, the SNP leader at Westminster who will co-ordinate his party's campaign, said: "It is the second poll in as many days to put backing for independence at about 40% and we are extremely confident that support will grow."

In the negotiations to come between London and Edinburgh on the referendum ground rules, having only a straightforward yes-no question on Scottish independence is a red line for Mr Cameron.

The belief is he will offer to accept Mr Salmond's timetable of autumn 2014 if, in return, the First Minister drops any "devo-max" option.

Thus far, the SNP Government has made clear its preferred option is a straightforward yes-no question.

Equally, the administration insists the "devo-max" option has a groundswell of support and should be considered as part of its three-month consultation exercise, which begins next week.

Ruth Davidson, the Scottish Conservative leader, said: "Poll after poll has consistently shown the majority of Scots want to remain part of the United Kingdom. Scotland is better off in Britain and Britain is great because Scotland is in it. It is now time this issue is settled once and for all."

Scottish Labour leader Johann Lamont said: "I will not hesitate to put Scottish Labour at the forefront of the debate about how we refresh and invigorate devolution for decades to come, but I want the people of Scotland to settle the choice we face – whether or not to separate – decisively."

Other findings showed 33% backed having the referendum in 2014, 23% said it should be this year while 15% said it should be in 2013 as Mr Cameron wants.

In a boost to Nationalist fortunes, the poll of more than 1000 adults also showed, compared to May last year, the SNP picking up support – which will worry Labour ahead of the May local elections.

On Westminster voting intentions, the SNP has leapt from 28 points to 37 while Labour has fallen from 44 to 35. On Holyrood, the SNP has risen from 42 to 44 points on the constituency vote, and from 35 to 39 on the regional vote. Labour has fallen in both; from 35 to 32 in the former and from 32 to 31 in the latter.

Mr Robertson said the poll put the SNP on its "highest ever YouGov rating for a UK election".

Earlier, Lord Wallace, the Advocate General, made clear the SNP Government's consultation paper could "not conceivably produce a solution to the legal problem, which prevents Holyrood holding an indepen- dence referendum".

The former LibDem deputy first minister, who leads for the Coalition in the Lords on legal matters, insisted the Scottish Parliament had no power to hold a lawful poll whether "advisory, consultative or providing a basis for negotiations". The Scottish Government disagrees.

Stressing important consequences flowed from this, Lord Wallace added: "One is that to proceed with a referendum that is outside of its legal powers would be to act contrary to the rule of law ... Government, according to law, is a fundamental principle of democracy; to flout this principle would be a very worrying step for a democratically elected government to take."

Meantime, the much-awaited meeting between Mr Cameron and Mr Salmond looks set for next month.

On Sunday, an ICM poll found 40% of Scots backed independence, while 43% wanted to retain the Union.