Celia Fitzgerald told the Sunday Herald she had been an SNP member since 2004 but had quit in November and then immediately joined Labour and LFI in order to agitate from within against Labour's support for the union.
An Alex Salmond fan who refers to "his Eckness" being a cuddly "honey" on her Facebook page, the 63-year-old former anti-apartheid campaigner from South Africa is now LFI treasurer and its organiser in Edinburgh and the Lothians.
The pro-union Better Together campaign last night said Fitzgerald's case proved LFI was a skin-deep Nationalist fake, not the important grass-roots development claimed by the SNP.
The SNP and Yes Scotland both welcomed LFI's formation in June 2012, citing it as evidence of a Labour split on independence, and a sign that Labour voters - who will be key to the result of the independence referendum - were open to voting Yes.
However, the group, which has struggled to gain momentum, has been dogged by claims it is little more than an SNP-inspired device to sow dissent inside Labour and trick the party's supporters.
The issue flared up last week after photographs emerged of three SNP councillors from Midlothian holding LFI banners and leaflets in the street.
The Sunday Herald has also obtained pictures of SNP councillors sitting with piles of LFI leaflets in a pub and manning an LFI stall.
LFI leader Allan Grogan said he is "aware that there are other examples of SNP members handing out Labour for Independence leaflets", and has admitted only 40% of his group's 80 or so members are actually in the Labour Party.
Grogan also said last week that his group had received financial support from the SNP-led Yes Scotland campaign.
Yes Scotland paid the £245 accommodation bill for the LFI's first conference last year.
Fitzgerald, from Dunbar, said she was in Scottish Labour for 30 years from 1974 but left after being disillusioned by the Iraq war. She conceded her journey from Labour to the SNP and then back to Labour was "curious".
She said: "I left Labour ages ago and joined the SNP to work for independence, then rejoined the Labour Party [in East Lothian] after I heard about Allan Grogan and Labour for Independence.
"I rejoined the Labour Party in November.
"It's about reclaiming the Labour Party for its members and its supporters and the people, and taking it away from this ghastly leadership."
She added: "New Labour is not real Labour at all. It's a Westminster party. It's not presenting the people of Scotland. We see it as a Scottish party, and we want to bring it back to Scotland, dust it down, return it to its proper values and then re-export it to England. We can't do that unless Scotland is independent."
She added that it was "ludicrous" to suggest LFI was an SNP front or dishonest in its activities.
LFI chairman Alex Bell, 59, who rejoined Labour in mid-2012 after many years out of the party, said he hated the SNP and was a "tactical nationalist" who saw independence as a way to socialism and "saving" the Labour Party from New Labour.
He said: "Getting rid of New Labour is the issue. If someone could convince me there was still a British road to socialism I might not be voting Yes. It [independence] frees the current Scottish Labour Party or a new one from having to pander to that Thatcherite greed of the southeast of England to get elected."
Better Together campaign director Blair McDougall said: "Last week, we had photographic evidence of senior SNP politicians attempting to deceive the public. Now it appears that there are deliberate attempts by the SNP to infiltrate the Labour Party to support the deception.
"There are serious questions to answer for the SNP, Yes Scotland and 'Labour' for Independence. How far up the nationalist chain of command did this deception go?
"We know Alex Salmond and Nicola Sturgeon have shared platforms with this group. Why did they work so closely with this organisation when it was clear 'Labour for Independence' was nothing more than a sham?"
Anas Sarwar, Scottish Labour's deputy leader, has denounced LFI as a desperate charade and called on Deputy First Minister Nicola Sturgeon to condemn SNP members taking part in LFI events.
An SNP spokesman said: "If an individual chooses to join the SNP from the Labour Party, and then chooses to go back to Labour again, that is a matter for them. All this highlights is that there are card-carrying Labour members who intend to vote Yes in next year's referendum.
"Labour would do better to worry about why they are 18 points behind the SNP in the polls."