Frustration is growing in Whitehall that Alex Salmond is "dragging his feet" on sorting out key issues surrounding the 2014 poll, most notably on whether there should be one or two questions.
To be able to deliver the SNP Government's preferred time- table, it is thought there is just a matter of months to pin down the technical details of the referendum. By next spring, if agreement has not been reached, then the Prime Minister faces a major political dilemma.
Asked if he might have to decide Westminster will legislate to hold an independence referendum in Scotland, a senior Coalition source told The Herald: "Potentially, this is a scenario he may have to face."
Another said: "Salmond is dragging his feet and it seems to us pretty clear why. It may well be the PM might have no choice but to make the decision himself."
Such a move would be fraught with political danger as it would leave Mr Cameron open to accusations from Nationalists that he is trying to hijack the process they feel is Holyrood's preserve. Ideally, the PM would not want to go down such a difficult path, but Whitehall sources have made it clear that, having come so far, Mr Cameron feels the people of Scotland deserve a "fair, legal and decisive" vote and that if only Westminster can facilitate it, then so be it.
UK Government ministers and officials are increasingly convinced the First Minister wants a second question, asking Scottish voters if they wish Holyrood to have extra powers beyond those set out in the latest Scotland Act.
In May, Michael Moore, the Scottish Secretary, called for urgent talks with Mr Salmond but has been told he will have to wait until the autumn when the responses to the Scottish Government's consultation are due to be published.
The expectation at Westminster is the First Minister will use the publication to justify supporting a second question because he fears he will lose a single one on independence.
Confidence of victory within the No camp appears high. One source said: "To be honest, at this stage we would have to try very hard to lose it. We would have to get complacent and we are not going to."
Talks have been going on in the background and David Mundell, the Scotland Office Minister, is due to meet Bruce Crawford, the Scottish Government's Parliamentary Business Secretary, to discuss referendum matters. However, it is thought any decisions will have to wait until Mr Salmond unveils the results of the consultation process sometime in September or October.
He would be attracted by the prospect of a near-certain victory in the referendum as, unlike on the subject of independence, opinion polls continually show most Scots would support a "more powers" option.
The referendum talks centre on the issue of a Westminster parliamentary order, which UK Government ministers insist is necessary to enable Holyrood to hold a legal poll given that Westminster is the UK's constitutional authority.
However, the so-called Section 30 Order is being offered on condition there is just one in- out question on the referendum ballot paper, a condition Holyrood is not willing to accept.
Mr Cameron is equally adamant he will not accept a two-question referendum, believing it would lead to confusion and could be challenged in court.
There have been suggestions Mr Salmond might, despite warnings from Westminster, simply decide to hold a Scottish Government poll anyway.
Professor Stephen Tierney, a constitutional expert he announced this week would act as his adviser on the referendum, said in January the outcome of a poll "generated from Edinburgh may be advisory only but the political impact of a yes vote would surely be irresistible".