FILE PHOTO: Pete Wishart MP (2ndL) and John Swinney MSP arrive at Perth Congregational Church in Perth, Scotland October 18, 2014. A private memorial service was held to commemorate David Haines. REUTERS/Russell Cheyne

EDINBURGH (Reuters) - A senior member of the Scottish National Party has questioned his party’s backing for a second vote on European Union membership, arguing that it could undermine its Scottish independence goal.

In a rare public sign of dissent Pete Wishart, an SNP lawmaker in Britain’s national parliament, said in a newspaper column that supporting a second “confirmatory” referendum could undercut the SNP’s goal of secession from the United Kingdom by inviting comparison.

Infighting in Prime Minister Theresa May’s Conservatives is complicating her efforts to reach a deal in time for Brexit day on March 2019. The SNP, Scotland’s biggest party, said this month it would back a second referendum on the issue -- although Scotland already voted to keep its EU membership in 2016.

Up to now the SNP had been one of the most unified parties on Brexit, supporting Britain’s continued membership of the single market and customs union.

“There is now a view amongst the politicians leading the “People’s Vote” (second EU referendum) campaign that all big constitutional referendums should now have a “confirmatory” second vote,” Wishart wrote in Scottish newspaper The National.

“By enthusiastically buying into this ‘confirmatory’ vote for an EU referendum we weaken our hand in resisting unionist calls for a second vote on a successful indyref (independence referendum).”

In the UK’s 2016 referendum on EU membership, a majority of voters in Scotland and Northern Ireland voted to remain in the bloc, while England - by far the most populous of the home nations - and Wales voted to leave.

May has consistently ruled out the idea of a second Brexit referendum while Labour, whose ranks are also divided, has dithered over the issue.