THE scale of the challenge facing Scottish Labour can be revealed after it emerged the party has one-sixth of the membership of the SNP.

Informed sources said the number of members currently sits at just under 13,500, a figure boosted by nearly 1000 new referendum campaign sign-ups.

Labour helped secure a No vote in September, but the party has been toiling due to the dramatic sequence of events in its aftermath.

Soaring memberships among the pro-independence parties have also confirmed that Scottish politics is in a state of flux.

The SNP tally has shot up from 25,000 to 84,228. The Scottish Socialist Party, which has long been in the political doldrums, has increased its membership from 1500 to 3500. The Scottish Greens had fewer than 2000 members before the referendum, but now have more than 7500.

Labour has consistently declined to confirm its total figure north of the Border, but it is understood nearly 13,500 members will get a vote in the contest to select Lamont's successor.

The party is choosing its leader in Scotland after the resignation of Johann Lamont, through a complex electoral college system which gives one-third of the votes to parliamentarians, one-third to affiliated organisations such as trades unions, and the remainder to party members. The system - which is to be phased out for UK leadership contests - means the vote of one MP or MSP carries the same weight as the votes of around 168 ordinary party members.

Labour's rules for the contest have also been designed to act as a recruitment tool for the party.

The usual six-month qualification rule - under which new recruits must have paid their dues for the specified period of time before being able to vote - has been waived. New members could join as late as Thursday and be eligible to vote.

Party sources believe the gulf in membership between Labour and the SNP will become apparent in the Westminster General Election campaign, when thousands of Nationalist supporters are expected to hit the streets.

MP Jim Murphy, one of the three Labour leadership contenders, wants to increase trades union sign-ups, involve non-party members in local campaigns, and use "reduced rate" initiatives to boost the figures.

A spokesman for Neil Findlay, a second leadership candidate, said he saw the trades union movement as key to future recruitment: "There is an obvious potential pool of support in trades unions and the party needs to focus and reconnect with the trades union movement. The relationship has to be deepened and strengthened."

A spokeswoman for Sarah Boyack, the third candidate for leader, said she wanted to target young workers and first-time voters, as well as targeting No supporters who don't have a natural party of choice.

SNP business convener Derek Mackay MSP said: "The extraordinary growth in SNP membership shows that the people of Scotland trust the SNP to hold the Westminster establishment to account on their vow of substantially more powers for Scotland."