Downing Street says he will publish his tax returns "as soon as possible"
David Cameron's admission that he owned, sold and made a profit on shares held in a controversial fund before he became British prime minister has prompted a Labour MP to call for him to resign.
John Mann - who has been a high-profile campaigner for more tax transparency - suggested Mr Cameron "has no choice but to resign" and called him a "hypocrite".
And Labour's deputy leader Tom Watson said the British PM may need to quit and has more questions to answer.
He told Sky News: "He has to be fully transparent. It's no good just saying he's going to publish his tax returns because that won't show what his investment portfolio was and we need to know that now."
Mr Watson added: "He's condemned other people in public life for being morally wrong whilst knowing he's had investments in these kind of schemes himself and that shows double standards and people don't like that."
Mr Cameron last night admitted for the first time he held shares in the fund set up by his late father.
The prime minister has faced questions this week over his family's tax affairs after details of Ian Cameron's investment fund were reported as part of the Panama Papers leak.
Number 10 has said he will publish his own tax returns "in the coming weeks".
In response to a question at an event on Wednesday, Mr Cameron said: "I have no shares, no offshore trusts, no offshore funds, nothing like that".
Now in his fifth statement on the matter, Mr Cameron - who has been a high-profile campaigner for more tax transparency - said he and his wife sold shares worth more than stg£30,000 (€37,077) in Blairmore Holdings before he became British prime minister.
The couple bought their holding in April 1997 for stg£12,497 (€15,445) and sold it in January 2010 for stg£31,500 (€38,931), according to Downing Street.
Mr Cameron said his and Samantha Cameron's profit was "subject to all the UK taxes in the normal ways" and it was just below the threshold at which capital gains tax would have applied.
The annual personal allowance for an individual in 2009-10 was stg£10,100 (€12,482) - meaning jointly the profit was just outside the threshold.
According to The Guardian, papers leaked from Panama-based law firm Mossack Fonseca are said to suggest Ian Cameron ran a fund that avoided having to pay tax in Britain by hiring Bahamas residents to sign its paperwork.
But Mr Cameron insisted it was a "fundamental misconception" that the fund was set up to avoid tax, saying his father was being "unfairly written about".
He also said he did not have "anything to hide" about his financial affairs.
He told ITV News: "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".
"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 also repeated he was willing to publish his own tax returns.
Among the revelations in the Panama Papers is a network of secret deals and loans worth US$2bn which apparently leads to Russian President Vladimir Putin.
Others said to be involved in the schemes include the prime minister of Iceland, who has now resigned after thousands called for him to quit in the wake of the scandal.