NICOLA Sturgeon has opened the door to a steep tax rise for Scotland's highest earners.

In the first televised leaders' debate of the election campaign, she said she wanted to increase the additional rate of income tax from 45p to 50p.

The First Minister ruled out the move when she unveiled the SNP's tax plans last week, warning there was a risk high earners might relocate to England reducing overall revenues for the Scottish budget.

Speaking during First Minister's Questions on Wednesday she described the move as "daft and reckless".

But she used the BBC's leaders debate, broadcast from Glasgow last night, to stress the plan would be kept under review.

She said: "I think there should be a 50p top rate of tax but you don't set tax rates if it's going to cost you money."

The additional rate is paid by around 17,000 Scots who earn over £150,000.

It will be kept at 45p in financial year 2017/18 if the SNP, as expected, returns to power after the May 5 poll.

Ms Sturgeon ruled out an immediate increase in the wake of civil service research which showed Scotland could lose £30 million if as few as seven per cent of top rate payers quit Scotland.

The risk appeared to rule out any realistic prospect of restoring the 50p rate in future years, despite a commitment to regular reviews.

But the SNP has come under pressure from Scottish Labour, which has promised a 50p top rate in addition to 1p increases to the basic 20p and higher 40p rates.

Outlining her party's plans, Kezia Dugdale, the Scottish Labour leader, said: "We can raise enough money to stop the cuts.

"Restoring the 50p rate was something Nicola Sturgeon used to stand for but it is something Labour would do."

Speaking after the debate, Labour's sole Scottish MP Ian Murray said: "Nicola Sturgeon was for it on Monday, against it on Tuesday and today she is not so sure.

"The SNP are all over the place."

Tax dominated the early exchanges in the debate, which also featured Scottish Conservative leader Ruth Davidson, Scots Lib Dem leader Willie Rennie, Scottish Greens co-convener Patrick Harvie and David Coburn, UKIP's MEP for Scotland.

Ms Davidson said she opposed any attempts to impose higher taxes in Scotland than the rest of the UK.

Asked by presenter Glenn Campbell whether the SNP's tax plans were "too timid", Ms Sturgeon said: "I've got Ruth Davidson saying I'm taxing rich people too much and Kezia Dugdale saying I'm not taxing poor people enough.

"I think that's a sign I'm probably in the right place."

The six leaders, who faced a live studio audience at the corporation's Pacific Quay headquarters, also faced questions on welfare, education, fracking and Donald Trump.

Mr Coburn struggled to explain his party's policy on welfare, appearing to confuse the issue with the health service.

Later, when the debate turned to education, he called for the return of grammar schools.

On fracking, Ms Sturgeon shifted further towards ruling out unconventional gas extraction, which has been subject to a temporary ban since the beginning of last year.

The Scottish Government has ordered a series of scientific studies, which are due to be completed next year, before making a long-term ruling.

Ms Dugdale, Mr Rennie and Mr Harvie all used the debate to repeat their promises of an immediate, all-out ban, arguing that exploiting a new source of fossil fuel would damage efforts to tackle climate change.

Defending the SNP's position, Ms Sturgeon said: "If there is any suggestion it harms the environment, it will not be allowed in Scotland."