Nicola Sturgeon's closest aide for more than a decade to vote for Alex Salmond's Alba Party

Nicola Sturgeon's closest aide for more than a decade has announced he is voting for Alex Salmond's new independence party in next month's Holyrood election.

Noel Dolan said backing the Alba Party on the regional list ballot on May 6 was "the best opportunity for getting a second independence referendum."

He said it would be "sensible" for nationalists to back their SNP candidates in their local constituencies then Alba on the list ballot paper, which assigns parties seats using a complicated form of proportional representation.

His intervention is deeply embarrassing for Ms Sturgeon as he was her most trusted aide for more than 10 years, before retiring in 2016.

He started to work for her in opposition when she was the SNP deputy leader, and Mr Salmond was leader, and became Ms Sturgeon's most senior special adviser when the party won power in the 2007 Holyrood election.

Mr Dolan gave his backing to the Alba Party ahead of the launch by Mr Salmond on Tuesday Afternoon of its national election campaign.

The former First Minister said he would unveil a "route to independence", with a strategy for "immediate action" following the May 6 election if there is a 'super-majority' of nationalists in the Holyrood chamber.

Mr Salmond has said he wants to manipulate Holyrood's complicated electoral system to deliver a "super-majority" for independence, with nationalists voting SNP in their constituencies and for his party on the regional list.

The SNP won 59 out of 73 constituencies in the 2016 election but only four on the regional list, which assigns seats to parties using a complicated form of proportional representation that deducts constituency wins.

An opinion poll at the weekend found separatist parties could together win 79 out of the Scottish Parliament's 129 seats on May 6, with the Alba Party winning six.

Mr Salmond has argued this would make it extremely difficult for Boris Johnson to ignore demands for another referendum.

However, Ms Sturgeon has accused Mr Salmond and his new party of trying to "game" the electoral system and warned this could backfire by reducing public support for independence.

Mr Dolan told the Daily Record: "I’m voting for what I regard as the best opportunity for getting a second independence referendum, or moving towards independence.

"On the basis of the way on which the system works it would be sensible, if you favour independence, to vote for the SNP in the constituency vote and Alba on the second vote.”

He said there is a “very strong probability” Alba will take seats off the unionist parties, therefore the “probability of an independence-supporting majority is increased by voting Alba”.

Speaking on Good Morning Britain, Mr Salmond insisted that Ms Sturgeon would work with him after the election to deliver independence despite her insistence she would not if he refused to show contrition for inappropriate behaviour towards women.

Mr Salmond has accused people in her inner circle, including her husband, of trying to remove him from public life and even imprison him.

Asked how he could work with his former protegee when "you can't stand each other", he replied: "Because independence for Scotland is a bigger cause than personalities."

He added: " "The jury was in a year ago and a predominantly female jury acquitted me of all the charges in front of a lady judge. The key point about that, is what holds us all to get onto a positive future for Scotland."

An SNP spokesman said: "The only safe way to ensure Scotland's future lies in Scotland's hands - not Boris Johnson's - is to give both votes to the SNP on May 6."