The admission comes five days after the leak of millions of secret documents from a Panama-based law firm which revealed that the prime minister's father, Ian Cameron, who passed away in 2010, ran an offshore firm under the name Blairmore Holdings.
After as many as four partial statements from Downing Street on the leak, Cameron confessed in a television interview last night that he did own shares in theBlairmore Investment Trust,which he sold for 31,500 pounds before he becamePrime Minister.
"I sold them all in 2010, because if I was going to become Prime Minister. I didn't want anyone to say you have other agendas, vested interests. Samantha (Cameron's wife) and I had a joint account. We owned 5,000 units in Blairmore Investment Trust, which we sold in January 2010. That was worth something like 30,000 pounds," he said.
He added: "I have been very clear about the future. I have said I am not going to benefit from any family trusts. I have been very clear about the present, I don't own any shares, I don't own any unit trusts or any investments like that.
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The Panama papers revelations have already sparked political reaction in several countries, including India, where high-profile figures have been mentioned.
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Earlier on Tuesday, Iceland's Prime Minister Sigmundur Gunnlaugsson became the first casualty of the leaks after he stepped down from his position following revelations that he owned an offshore company with his wife but had not declared it when he entered the Parliament.
According to ICIJ, Cameron's father Ian used Mossack Fonseca's services to shield profits from his investment fund, Blairmore Holdings Inc, with a series of expensive and complicated arrangements.
The prime minister had been under increasing pressure as he has refused to give details of his family's money held offshore in a Caribbean tax haven.
"It has been a difficult few days, reading criticisms of my father and his business practices. I think a lot of the criticisms are based on a fundamental misconception, which is that Blairmore, a unit trust, was set up with the idea of avoiding tax.
"It wasn't. It was set up after exchange controls went so that people who wanted to invest in dollar denominated companies could do so," Cameron told 'ITV News'.
The party feels Cameron's admission had failed to end the issue and demanded full disclosure on what other financial arrangements he had benefited from as an MP and then Leader of the Opposition, Watson added.