Q&A: Scottish independence row
Scotland's First Minister Alex Salmond says his government has a mandate to hold a referendum on independence in the autumn of 2014.
But the UK government says this would be unlawful without the UK Parliament's approval.
Here are some of the key questions about the row - for more on the background of the quest for Scottish independence click here.
What is Alex Salmond saying?
Mr Salmond's Scottish National Party (SNP) - which favours independence - won the Holyrood parliamentary elections last year pledging to hold a referendum during this parliament, which ends in 2016.
The first minister says this victory means he has a mandate to call a vote without asking permission from Westminster - and he is set on a date in the autumn of 2014.
That will be the year of the 700th anniversary of the Battle of Bannockburn - a victory of the Scots over the English - and of Glasgow hosting the Commonwealth Games. It is thought this could lead to an increase in pro-independence sentiment.
Others believe Mr Salmond favours a date as close as possible to the UK general election - currently scheduled for 7 May 2015 - at a time when Westminster parties arguing for the Union will be pitted against each other.
A final reason for waiting may be that Mr Salmond hopes the economic situation will have improved by then and Scots are far more likely to back independence if they feel confident about their future.
Whatever the date, even if the referendum is non-binding the SNP says its results will exert huge "moral and political force" over the UK government.
What is the UK government's opinion?
It does not want Scottish independence.
It also thinks it would be unlawful for the Scottish Parliament to hold any referendum on the issue - be it legally binding or advisory - without its approval.
The UK government says constitutional questions are "reserved matters", meaning they are for Westminster - rather than devolved administrations around the UK - to decide.
But the UK government says it is willing to offer a temporary transfer of the power to hold a referendum to the Scottish government.
However, it wants a referendum to be held "sooner rather than later", in contrast to Mr Salmond's plans for 2014.
So, does Scotland need Westminster's permission to hold a referendum?
The Scotland Act 1998, which brought about devolution, says that in deciding whether powers are reserved, attention must be paid to the "purpose" and "effects" of legislation from Holyrood.
Westminster says that as the purpose and effect of any referendum - even a non-binding one - is to hasten Scotland's exit from the UK, it is clearly a reserved matter.
But Alex Salmond argues there is "plenty of legal authority" to support his proposal to pass a law in Scotland.
How much support is there for independence in Scotland?
Polling expert John Curtice says support for independence is somewhere between 32 and 38% - down from where it was at the start of the SNP's last term in office as a minority government.
A YouGov poll conducted in April 2011 put support lower than that - at 28% - with 57% opposed.
But research suggests that the section of Scottish society vehemently opposed to independence - and who therefore refuse to vote for the SNP under any circumstances - has shrunk considerably in recent years - something which was a key factor in Alex Salmond's 2011 election victory.
That doesn't, however, mean that those people are now pro-independence, so how they would vote in a referendum remains to be seen.
What would be on the ballot paper?
Westminster favours a straight yes/no vote on independence. It wants to rule out a third option - so-called "devolution max" - which would offer more powers for Scotland, short of full independence. Unionists fear this will be harder to defeat because it will split their vote and win over those who otherwise would have said no to full independence.
In 2010 the SNP defined "devo max" as meaning control for Holyrood of all laws, taxes and duties - all revenue raising and spending activities - including control of the oil reserves in Scotland's waters. Defence, foreign affairs, financial regulation, monetary policy and currency would continue to be run from Westminster.
The SNP says it favours a simple yes or no vote, but accepts there is "a significant body of opinion" in Scotland which wants more powers, but not independence.
It has therefore proposed two questions along the lines of 1) Do you want devolution max? and 2) On the basis that you want devolution max, do you also want independence? The SNP says the two questions run in sequence and if both are endorsed - say 80% for the first and 51% for the second - the result will be independence.
But others would suggest that in fact the questions are in competition with each other - and plenty of people might want the former instead of latter - so a larger vote for devo max should mean that option, rather than independence, wins.
Who would take part?
Alex Salmond wants 16 and 17-year-olds, seen as more pro-independence than other age groups, to have a vote in the referendum.
But the UK government favours allowing only those above the age of 18 to vote - as in elections.
Can a deal be reached?
Alex Salmond has accused David Cameron of trying to stamp his "size 10 boots" over the will of the Scottish people and adopting a "London knows best" approach.
But the Scottish Secretary, Michael Moore, has made conciliatory noises - saying the UK would give the Scottish government the power to hold a binding referendum, if they talk to each other about how it is staged.
Mr Salmond said he is sure an agreement can be reached this year as long as it is recognised that the referendum must be "built in Scotland and decided by the Scottish people".
BBC political editor Nick Robinson says that, if the matter is not resolved, it could lead to an historic Supreme Court battle between Westminster and Holyrood.
Who could lead the unionist campaign?
Edinburgh South West MP and former Labour chancellor Alistair Darling emerged as an early front runner, but he has ruled himself out. He says he will play his part but does not want to take charge.
Scottish Labour leader Johann Lamont has said she is willing to lead, but her newness to the job may count against her.
The consensus among commentators is that whoever it is, they must be a Scottish, rather than Westminster, figure and cannot be too closely associated with one particular party.
Some have suggested a figure from outside the world of politics altogether - and the name of Manchester United manager and former Glasgow Rangers player Sir Alex Ferguson has even featured.