Doorstep chats with more than 18,000 voters in 90 working-class communities across the country showed almost two-thirds of voters in favour of independence once the undecideds were stripped out.
The Radical Independence Campaign (RIC), which carried out the survey, said it was the largest public canvass sample in the referendum to date.
And the findings are in stark contrast to official polls, which almost all show a No lead.
The RIC snapshot found support for Yes at 44 per cent, with No on 25 per cent and Don’t Knows on 31 per cent.
But if the undecideds are removed, the split is 63 per cent for Yes and 37 per cent for No.
One of the areas highlighted by RIC is Bonnyrigg in Midlothian, where child poverty is running at 15 per cent. The survey found Yes support at 46 per cent, No 25 per cent and Don’t Knows 29 per cent. Removing the undecideds gives figures of Yes 65.5 per cent, No 35.5 per cent.
Jonathan Shafi, co-founder of RIC, said: “This vote is not coming through in any official poll because they are not talking to people in these areas,
“These people have been told for years their views don’t matter. No-one has come to their door for ten years or more.
“What we wanted to do was look at the lowest turnout areas and ask ‘Where are the missing million?’ and actually go and speak to them. Our belief is the vast majority of these Don’t Knows are not just moving towards Yes, they will vote Yes on polling day. These are Labour strongholds. We are absolutely confident that if we can mobilise these votes we can win.”
Robin McAlipine, director of the Jimmy Reid Foundation, agreed many Labour voters were backing independence. He said: “The No campaign has already lost traditional working-class Scotland. Traditional Labour heartlands are breaking from the official Labour line.”
A YouGov poll on Monday showed support for independence increasing, but still gave the No campaign a clear lead with 57 per cent support to 37 per cent for Yes, a gap of 14 points.
However, the Economic and Social Reserach Council, based at Edinburgh and Stirling universities, has also found support for independence split on class lines with 46 per cent of those on lowest incomes in favour, compared with just 27 per cent of the highest earners.
It said: “The higher the position on the social scale, the lower the support for independence.”