A second referendum over whether Scotland should leave the UK is just weeks away from being called, a leading Scottish independence campaign group has said.

Business for Scotland (BfS) has begun a fundraising drive and says it is getting "back in full campaign mode" for an expected "Indyref2" vote in May or September next year.

First Minister Nicola Sturgeon, who backed the BfS crowdfunding appeal with a video message, has previously said a second referendum was "highly likely" following parliament's vote to trigger Article 50.

In his message to supporters, BfS founder Gordon MacIntyre-Kemp said: "Business for Scotland was a vital contributor to the growth of the Yes vote in 2014. We need your help to renew the business and economic case for independence and drive Yes support to the levels required to call and win Indyref2.

"We didn't go away, instead we have kept on campaigning for independence and we also campaigned for a Remain vote in the EU referendum.

"We are getting ready for Indyref2 in May or September 2018 and so we are back in full campaign mode."

MacIntyre-Kemp added in an email to members that "we are only weeks away" from a second referendum being called, the Sunday Herald reported.

The BfS, which was a key component of the Yes movement during the 2014 vote, has already raised nearly a third of its £15,000 ($18,600) crowdfunding target.

The Scottish government is yet to comment on the lobby group's claims.

Prime Minister Theresa May said earlier this month the Scottish independence referendum was "settled" by the first vote, adding: "Recent polls don't suggest that there has been a big change in the views around a second referendum."

The SNP argues that since Brexit was opposed by a majority in Scotland, the decision to leave the EU warrants a second vote – something the party would need permission from the UK government to hold.