By John Geddie and Elisabeth O'Leary

LONDON/EDINBURGH (Reuters) - The Scottish government is increasingly confident it can win a new independence referendum and is considering calling one next year as Britain exits the European Union, sources close to the Edinburgh administration say.

Scots voted overwhelmingly to stay inside the EU in last June's referendum, but Britain as a whole voted to leave, taking Scotland with it. British Prime Minister Theresa May plans to trigger the two-year Brexit process next month.

"I believe the Scottish government is thinking very, very seriously about going for an independence referendum next year," Charles Grant, an adviser to the Scottish government's Standing Council on Europe, said on Thursday.

"They feel they have enough emotion and momentum to overcome the economic downsides ... the harder the Brexit, the more likely they are to break away."

Scots rejected independence by a 10-point margin in a 2014 referendum, with many voters expressing concern about how Scotland's economy would fare after leaving the UK.

The British government says there is no need for a second independence referendum.

But Scottish First Minister Nicola Sturgeon said the circumstances this time are completely different, as Scots who voted to stay in the UK were voting to be part of an EU country, something that was no longer the case.

"We have made it very clear that an independence referendum is very much an option on the table if it becomes clear that it is the best or only way to protect Scotland's vital national interests," the Scottish government said in an emailed comment for this article.

Sturgeon published a draft bill for a second independence referendum last October. The divorce of Scotland and England, severing a 300-year-old trading, cultural and political union, would come on top of Britain's traumatic separation from the EU.

Scottish ministers say a proposal made in December for a separate Scottish deal within Brexit has not been seriously considered. The UK government denies that is the case.

One Scottish lawmaker, speaking on condition of anonymity, said: "If you don't call (an independence vote) now, it's off the cards for a generation," because economic damage from Brexit would make voters nervous of more change.

Ross Greer, a Scottish Greens lawmaker whose party supports Sturgeon's independence goal, told Reuters he had "substantive discussions with various key people" which lead him to conclude a referendum would be called for autumn 2018.

The "Remain" side would be weaker this time as it could no longer tell voters to choose a certain status quo, he said, as Brexit means years of uncertainty about Britain's trading relationships.

"The challenge is to agree an appropriate time for the vote itself and that involves Westminster as well," he said. A new referendum would have to be approved by the UK parliament in London.

On Wednesday, UK Secretary of State for Scotland David Mundell told the Scottish parliament the country would have to leave the EU whether or not it became independent, angering nationalists.

(Additional reporting by Marc Jones; Editing by Andrew Roche and Robin Pomeroy)