Scottish independence: RMT members back 'Yes' vote
The RMT has become the first major industrial trade union to come out in favour of Scottish independence.
The transport union said a ballot of its Scottish members had seen them "narrowly" record a majority in favour of independence.
The result came as Labour leader Ed Miliband was campaigning in Scotland for a "No" vote.
The RMT was disaffiliated from Labour in 2004 after it allowed its branches to affiliate to other parties.
It is understood RMT members in Scotland voted in favour of independence by a margin of 1,051 to 968. A further 365 voters were undecided.
The RMT is the largest of Scotland's rail unions, and also represents many offshore oil and gas workers.
A spokesman for the union said: "RMT has conducted a referendum ballot of our members in Scotland and they have narrowly recorded a majority in support of the 'Yes' position.
"That ballot result has been reported to the union's executive and we will be sending out a formal letter to our members in Scotland in due course."
Welcoming the result, Deputy First Minister Nicola Sturgeon described it as a "humiliation" for the Labour leader.
She said: "This is very welcome and hugely significant backing for a 'Yes' vote from members of a major trade union - and it is a humiliation for Ed Miliband.
"Coming on the same day as his latest visit to Scotland, it completely demolishes his claims to be the authentic voice of working class, Labour-minded voters across Scotland.
"Labour voters are already moving to a 'Yes' vote in their hundreds of thousands, and it is endorsements like this which will persuade even more of them to back independence, as we look to securing a majority of Labour supporters voting 'Yes' on 18 September".
A spokesman for Scottish Labour said: "Six trade unions, representing 140,000 people across the public and private sector in Scotland, are in favour of a 'No' vote on 18 September.
"The vast majority of trade unionists in Scotland agree that we are stronger together and want to side with the politics of unity and solidarity, and not the politics of division.
"Even this ballot did not show a majority in favour of independence - 59% of people who voted didn't support a 'Yes' vote."
The Prison Officers Association Scotland has also previously voted in favour of independence.