A SECOND nationalist MP has admitted breaking strict Westminster rules by failing to declare a shareholding in a private company.

Brendan O'Hara, the SNP's defence spokesman, said he had made a "mistake" by not disclosing his 50 per cent stake in Oh! Television Limited, which the former TV producer set up in 2009 after leaving his job with the BBC.

It comes after parliamentary standards authorities launched an investigation into Phil Boswell, the SNP MP for Coatbridge, Chryston and Bellshill, who has admitted that he failed to declare his shareholding in the firm Boswell and Johnston Ltd.

Mr O'Hara said he had been initially unaware that he was obliged to record his interest in Oh! Television as it had never traded or made any money. He said he had established the company in case he did not receive enough freelance work after leaving the BBC, but that it had remained dormant ever since he set it up.

However, rules state that MPs are obliged to declare any shareholdings over 15 per cent, even if a company is not trading, on their register of interests.

Company records show that the Argyll and Bute MP, who won a majority of more than 8,000 at last year's general election, signed off dormant Oh! Television accounts in August - three months after he was elected.

Mr O'Hara said: "It's never traded, it's never earned a penny. It's my oversight. There is nothing secretive, it's never traded in its life. I was quite naive to think that this didn't count.

"I had conversations with the whips office and on the first day back after recess I phoned the register office and they said 'yes, although it's never traded it technically should be on'. There was never an attempt to try and hide this or any earnings. It's a non-story."

He said that the SNP had given its new cohort of MPs advice on what should be declared on the register of interests following its landslide win last May and added: "I don't blame the party. I wouldn't blame anyone other than myself. As a new MP there are a million and one things you have to do."

While Mr Boswell is subject to an investigation from Kathryn Hudson, the parliamentary commissioner for standards, there is currently no probe into Mr O'Hara and he said Westminster authorities had accepted that the failure to declare the shareholding had been a genuine mistake.

SNP MPs Michelle Thomson and Natalie McGarry have both left the party following allegations of financial irregularities and while Mr Boswell is expected to retain the party whip, the standards probe into him as well as revelations that he previously benefitted from a tax avoidance scheme have been a source of embarrassment to Nicola Sturgeon.

Murdo Fraser, the senior Scottish Conservative MSP, said: "At this rate there won’t be any SNP MPs left by the end of the parliamentary term who haven’t run in to some sort of trouble. This is another case of the SNP claiming to be whiter than white, but the evidence proving quite the reverse."

Mr O'Hara, whose great, great grandfather was one of the co-founders of Celtic, caused controversy ahead of the general election after it emerged that he had used the word 'hun' to describe Rangers supporters on an online forum under the name 'spikeybhoy'. He apologised for any offence caused but resisted calls to stand down as a candidate after a petition was launched calling for the move.