NICOLA Sturgeon went eyeball to eyeball with David Cameron when the pair met on Friday and told him he would be personally responsible for a second independence referendum if more powers did not flow to Scotland, the Sunday Herald understands.

The First Minister used a private moment immediately before formal talks at Bute House on Friday to warn the Prime Minister that his actions - or inaction - would determine whether Scots demanded a second vote on the country's future.

If Cameron failed to heed the clamour for more powers, as seen in the election of 56 SNP MPs, then he should expect another referendum in a few years, Sturgeon said.

"The ball is in your court," she told him, according to Scottish Government sources, who described Sturgeon's message as "very, very blunt".

It is understood Cameron disagreed and said how both of the leaders acted would determine what happened.

The warning was delivered in the Scottish Cabinet room during a 10-minute one-to-one before Sturgeon and Cameron held their first official meeting since the election.

During the 40-minute formal session, which was also attended by officials and Deputy First Minister John Swinney, Cameron pledged to implement the Smith Commission proposals on further devolution in full through a new Scotland Bill.

The plans - which include Holyrood controlling income tax, a half share of VAT and some benefits - flow from the Unionist parties' pre-referendum "Vow".

Cameron later said he would also "look at" the SNP's demands for yet more powers, but ruled out full fiscal autonomy as "not a good option".

Sturgeon said the meeting had been "constructive and helpful", but she would continue to press for Holyrood to have control of business taxes, employment law, the minimum wage and welfare in order to grow the economy and alleviate poverty.

However, a source close to the First Minister said the more pointed discussion came when Sturgeon and Cameron were alone at the start of proceedings.

"Cameron was told the ball is firmly in his court - so how things develop in Scotland over the next couple of years or so is now mainly up to him.

"The Tories can recognise the significance of the mandate flowing from the General Election and work to deliver more powers, or they can go on pretending it is business as usual and watch as support for independence grows."

The warning appears to be a shift away from two of the SNP's messages before the election - that the result would have no bearing on the timing of a second referendum, and that a "material change in circumstances" was a pre-condition for another vote. Until now, the latter has been thought to refer to a dramatic event, such as Scotland voting to stay in the EU while England votes to leave, but Sturgeon's warning implies the SNP could cite an underwhelming Scotland Bill as the trigger.

Former deputy SNP leader Jim Sillars made a similar point last week in the Sunday Herald, saying of Cameron: "Everything depends on him.

"If Cameron doesn't handle this properly, and we're back in the old trap of having rejected an English Tory government and got it ruling us, the only way out of that trap is independence."

Cameron's team also tried to avoid meeting Sturgeon at Bute House, her official Edinburgh residence, and wanted to meet in a dull government building instead.

"Nicola went to see him in Downing Street after she became FM, so it was only right that he should come to our equivalent of Number 10," a government source said.

A Downing Street source said: "The SNP were signatories to Smith and we've made it clear we will implement Smith. The Prime Minister also said he would look at ideas. The SNP doesn't know where it stands. Every day it's a different message."

The Royal Society of Edinburgh will this week warn MSPs the Smith proposals are being rushed, undermining the chance of an enduring settlement.

In evidence to Holyrood's finance committee, the RSE says more time is needed to consider future risks, a new fiscal framework and changes to the Barnett Formula.