FILE PHOTO: The SNP's deputy leader Angus Robertson campaigns in Elgin, Moray, Scotland, Britain May 18, 2017. Picture taken May 18, 2017. REUTERS/Russell Cheyne/File Photo

LONDON (Reuters) - Angus Robertson said on Saturday he was standing down from his role as deputy leader of the Scottish National Party (SNP) with immediate effect, after he lost his seat in the British parliament last June.

“I believe I am no longer able to fully discharge my mandate, which was to partner you as Westminster SNP Leader and as a parliamentarian representing a rural constituency,” he wrote in a letter to Scotland’s First Minister Nicola Sturgeon that was posted on Twitter.

Robertson led the SNP’s group in the British parliament until losing his seat in June’s snap election, as some voters in Scotland switched away from the party in opposition to a second referendum on independence from the United Kingdom.