Holyrood 2016: Labour's Dugdale quizzed over SNP work experience claim
Scottish Labour leader Kezia Dugdale has played down suggestions she applied for work experience with the SNP 13 years ago.
She told the BBC it was "laughable" that a "supposed" application made in February 2003 should be central to the Holyrood election campaign.
The Lib Dems called for a probe into how the application was "leaked".
The SNP said "no documents of any kind" had ever been given to "any outside organisation".
Party leader Nicola Sturgeon added that no-one "should be judged too harshly" on what they did in their youth.
Parties are in the second week of campaigning ahead of 5 May poll.
The news was first reported in the Sun newspaper, which suggested Ms Dugdale had applied for work experience with SNP MSP Richard Lochhead.
When asked by the BBC's Glenn Campbell whether she had applied for work experience with the SNP, Ms Dugdale said: "Thirteen years ago I supposedly applied for work experience in the Scottish Parliament.
"Now I applied for lots of work experience, lots of young people at university do just that.
"But the idea that this is a central issue of the Scottish election campaign is really laughable."
The 34-year-old repeated: "This is 13 years ago. In February 2003, apparently. I was studying law, I likely applied for lots of opportunities for work experience."
Ms Dugdale said she had never voted SNP and had never been an SNP supporter.
Scottish Liberal Democrat leader Willie Rennie said whether or not Ms Dugdale applied for work experience with the nationalists was "largely trivia".
But he added the SNP had "serious questions" to answer over how the information had become public, saying it should have been kept private.
He later wrote to the Information Commissioner, calling for an investigation into what could be "a serious breach of data protection".
Labour's Scottish MP, Shadow Scottish Secretary Ian Murray, called on Nicola Sturgeon to "condemn" the leak and "promise an end to dirty tricks in this election."
He said: "The story itself was laughable but data protection is no laughing matter. People need to know that their correspondence with an MSP will be kept private, not leaked to a national newspaper for petty party politics.
"Richard Lochhead is a cabinet minister. He was named in this story by The Sun and neither he or the SNP can avoid questions on this."
A spokesman for the SNP responded: "The fact Ms Dugdale asked for a position in the SNP has been very common knowledge in Holyrood circles for a long time.
"No documents of any kind have ever been given to any outside organisation."
Ms Sturgeon, who was out on the campaign trail in Edinburgh, was asked about the work experience claim.
She joked that her opponent may have had better judgement when she was 21.
However, Ms Sturgeon went on to say that people should not be judged too harshly on what they did in their youth.
Scottish Conservative leader Ruth Davidson questioned whether anyone could trust Labour with a leader that "keeps flip-flopping on this most vital issue".
The pro-Union politician added: "Firstly, Kezia Dugdale says she's happy for Labour MSPs to campaign for independence, then she says it's 'not inconceivable' she'd vote for it herself and now we find out she even asked the SNP for a job - twice."
Last week Ms Dugdale was forced to clarify reports she might back Scottish independence if it could secure Scotland's EU membership.
In an interview with the Fabian Review she was quoted as saying it was "not inconceivable" in such circumstances.
But she later insisted she would vote to stay in the UK in any future referendum and opposed one being held.