British Prime Minister David Cameron said Scotland should hold an independence referendum as early as next year, clashing with the Scottish National Party, which wants more time to rally support for a break from the UK.

Cameron, who opposes Scottish independence, said uncertainty about the 300-year-old union between England and Scotland was creating problems for business and harming investment.

"If [SNP leader] Alex Salmond wants a referendum on independence, why do we wait until 2014?" Cameron told Sky News.

"This is very damaging for Scotland because all the time business is asking: 'is Scotland going to be part of the UK? Are they going to stay together? Should I invest?'"

Scotland, which kept its own legal system after the 1707 union, has had a devolved government since 1999, with control over health, education and prisons.

The SNP won an overall majority in Scottish elections last May and has promised to hold a referendum in the second half of a parliamentary term lasting until 2016.

The party is hoping to exploit two events in 2014, the Commonwealth Games in Glasgow and the 700th anniversary of the Battle of Bannockburn when the Scots vanquished an English army, to create momentum for a breakaway.