Supporters of Scottish independence have gained ground ahead of the referendum in two months' time by snapping up more undecided voters.
According to British Election Study (BES) data, which tracks individual voting intentions, the gap is closing between the 'Yes Scotland' campaign and the 'Better Together' camp, but this is still not enough to break Scotland away from the 307-year union with England.
"The BES is unique in that it is able to track the referendum vote intention of the same voters over time," said Professor Ed Fieldhouse from The University of Manchester, who co-directs the study with other universities, Oxford and Nottingham.
"And the data tells us that the vast majority of those intending to vote Yes or No did not change their mind in the three months between March and June. However, the 'don't knows' are still significant. We find they are more likely to vote 'Yes' when the time comes.
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"But our data also shows not enough of them will support the Yes camp to win the eventual race: more people are yet to be persuaded."
Scots will vote in an independence referendum on 18 September this year and will be asked the straight "yes/no" question: "Should Scotland be an independent country?"The referendum period started on 30 May.
BES collected the voting data initially between 20 February and 9 March, and then in a second wave between 22 May and 18 June. The data showed that the voting gap has started to close between the first and second waves of voters.
"June's data shows little change with the 'No' vote at 51%, the 'Yes' vote at 39%, with 10% of the sample still undecided and the rest saying they will not vote," said BES.
"Findings from the first wave showed the No camp ahead by 52% to 37% with the rest undecided."
In March, 25% of people who had not decided which way they would vote ended up supporting the 'Yes' campaign by June, compared with 18% who voted 'No.'
BES said that, overall, nearly 90% of voters did not change their mind in the three months between the two waves.