David Cameron said he did not have "anything to hide" about his financial affairs as he revealed that he and his wife sold shares worth more than £30,000 in an offshore tax haven fund set up by his late father.
The Prime Minister has faced intense pressure to detail his interests since the Panama Papers leaks included details of Blairmore Holdings - which used "bearer shares" to protect investors' privacy.
In an interview with ITV News, he insisted that it was a "fundamental misconception" that it was set up to avoid tax, saying his father was being "unfairly written about".
And he said that while his and Samantha's profit from the scheme was "subject to all the UK taxes in the normal ways" - it came to just below the threshold at which capital gains tax would have applied.
Mr Cameron, who has been a prominent campaigner for increased tax transparency, also repeated his willingness to publish his own tax returns.
Number 10 said Mr and Mrs Cameron bought their holding in April 1997 for £12,497 and sold it in January 2010 for £31,500.
The annual personal allowance for an individual in 2009-10 was £10,100 - meaning jointly the profit was just outside the threshold.
Downing Street had initially said it was a private matter before first clarifying that the PM had no offshore funds and trusts and then making clear the family would not benefit in future either.
"I paid income tax on the dividends, but there was a profit on it but it was less than the capital gains tax allowance, so I didn't pay capital gains tax, but it was subject to all the UK taxes in all the normal ways," Mr Cameron told ITV.
"So I want to be as clear as I can about the past, about the present, about the future, because frankly, I don't have anything to hide.
"I'm proud of my dad and what he did and the business he established and all the rest of it.
"I can't bear to see his name being dragged through the mud, as you can see, and for my own, I chose to take a different path from my father, grandfather and great-grandfather, who were all stockbrokers, and I've got nothing to hide in my arrangements and I'm very happy to answer questions about it."
Mr Cameron said it had been "a difficult few days" since the mass leak of records from Panamanian law firm Mossack Fonseca as he launched a staunch defence of his father.
"I think a lot of the criticisms are based on a fundamental misconception, which is that Blairmore Investment, 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 shares and companies could do so, and there are many other, thousands of other unit trusts set up in this way.
"It was reported to the HMRC, the Inland Revenue. It reported itself every year. It was properly audited.
"It wasn't a family trust. It wasn't for the benefit of one particular family; anyone could have bought units in it. And crucially, if you were a UK citizen and bought units in it, then you paid income tax on the dividends, and you paid capital gains tax when you sold the shares.
"So you're subject to full UK taxation.
"So that's what it was. There are many other unit trusts like it, and I think it's being unfairly described, and my father's name is being unfairly written about.
"But look, I'm very clear about my situation. I'm happy to answer questions about it. And above all, I'm going to go on fighting very hard to make sure we have greater transparency, greater rules on tax avoidance, and aggressive tax evasion and aggressive tax avoidance. It's very important that we do.
"Now, I've been very clear about the future. I've said I'm not going to benefit from any family trusts. I've been very clear about the present: I don't own any shares, I don't own any unit trusts or any investments like that. I own two homes, one of which I rent out, and I have a salary as Prime Minister. So my affairs are very transparent. Happy to make them more transparent.
"But I should deal with the past as well, because of course, I did own stocks and shares in the past, quite naturally, as my father was a stockbroker.
"I sold them all in 2010, because if I was going to become Prime Minister, I didn't want anyone to say you've got other agendas or vested interests or all the rest of it. Samantha and I had a joint account, and we owned 5,000 units in Blairmore Investment Trust, which we sold in January 2010. That was worth something like £30,000."
Mr Cameron said he was not able to "point to every source of every bit of the money" that his father left him when he died - which he said amounted to around £300,000.
Pressed on whether he benefited from any part of the estate that was based in Jersey, he said: "Dad's not around for me to ask the questions now. But he was a very hardworking man. He built up a business. He left his house to my brother. He left me some money, and left things to my brothers and sisters, too.
"And I think, you know, there's pretty good transparency about all of that. And as I've said, in the future I'm not benefiting from any Cameron family trust. At the moment I own no shares, no investments. I have savings."
Mr Cameron said in 2012 that he was happy to publish his own tax return - but the idea has not been put into practice.
He told ITV that he remained relaxed that it should be the norm for prime ministers - and leaders of the opposition - to have their status opened to scrutiny.
"I'm happy to, you know, go and get on and do that now. Whether we do it, you know, every year or starting now or going back a couple of years. Whatever. I'm very happy with that.
"And that will be a big change so we shouldn't, you know, dive into it and suddenly insist everyone has to do it. But I think if people really want prime minister, potential prime minister, as I said before, I'm relaxed about that and I'm happy to do that."
Mr Cameron's father Ian, who died in 2010, was a director of Blairmore Holdings Inc, which, until 2006, used unregistered "bearer shares" to protect its clients' privacy.
His use of the firm to help shield investments from UK tax helped build up a significant legacy, part of which was inherited by the PM.
There is no suggestion that this avoidance arrangement or others exposed by the leak were anything but entirely legal.
Labour's John Mann insisted Mr Cameron should resign and labelled the PM a "hypocrite".
The Treasury Select Committee member wrote on Twitter: "Cameron has been less than honest. He should resign immediately. Most decent people would expect nothing less."
Mr Mann, MP for Bassetlaw, added: "Cameron has had six years to be honest with Parliament and the people. He failed to do so. Get out now hypocrite.
"Cameron issue is simple. He covered-up and misled. How he got his shares is irrelevant. He has no choice but to resign."