The Scottish National party has voted to establish an independent currency “as soon as practicable” if Scotland were to leave the UK, as activists rejected a more cautious timetable put forward by the party leadership.

In a narrow victory for the grassroots, the party conference agreed by 52 votes to amend part of a wide-ranging economic blueprint, created by the party’s sustainable growth commission, to allow for a new currency to be fast-tracked and “ready for introduction as soon as practicable after Independence Day”.

The SNP leader and first minister, Nicola Sturgeon, had urged the membership to support a more gradual approach before this weekend’s campaign conference in Edinburgh.

The amended resolution was passed following a tense but respectful debate, which party deputy Keith Brown urged should be “an advert for independence”.

The growth commission’s report, which was published last May, attracted a wave of criticism for what many saw as an overly cautious and market-driven approach, including its plans to keep sterling for an indefinite period while cutting public spending.

It faced a backlash on the conference floor with a number of amendments tabled, several of which attempted to scrap the “six tests” on market credibility, trade and public opinion that the commission recommends should be met before a new currency is established.

Delegates did not support scrapping the six tests, but amended the wording relating to when a new currency could be established, removing the defined timescale that a decision should be taken by the end of the first term of an independent parliament.

Proposing the resolution along with the Scottish government’s finance secretary, Derek Mackay, Brown told delegates that a convincing economic policy was key to strengthening the case for independence.

Reminding the conference hall in Edinburgh that they could not take for granted that support for independence would increase, he recognised the urgency that some felt about establishing a new currency but warned that “going harder and faster would damage that case and fail to win more support”.

Brown warned them: “When you cast your vote think about how you want to feel the day after another referendum.”

Proposing the amendment to scrap the tests, the SNP veteran and former MP George Kerevan insisted that the debate was not about “a divide between those who want to convince and those who are rushing ahead”, but argued that voters on the doorstep would want to know when the party planned to introduce a new currency, and the six tests would lose support.

As the proceedings unfolded, some speakers compared the atmosphere to the 2012 debate when the party changed policy to remaining within Nato as an independent country, against fierce grassroots opposition.

But most party members and politicians were in buoyant mood, as the sunshine streamed in through the high windows of Edinburgh’s International Conference Centre, following a YouGov poll released overnight that found support for independence up four points since last June to 49%.

The survey, the first to be conducted since Sturgeon said she would introduce legislation to allow the option of holding another independence referendum before 2021 if Brexit goes ahead, also found that 48% thought there should not be a second vote within the next five years.

It also put Nigel Farage’s Brexit party in third place in European election voting intention at 13 points, behind Labour at 14 and the SNP at 40 points, with the Scottish Conservatives trailing at 10.