David Cameron must take "real action" to crack down on offshore tax havens, opposition figures have demanded after a massive data leak exposed the scale of efforts by the rich and powerful to hide assets.
The Prime Minister's late father was reported to be among names - including those of six peers, three ex-Tory MPs and political party donors - named in relation to investments set up by Panamanian law firm Mossack Fonseca.
Downing Street said it was a "private matter" whether the Cameron family still had funds in offshore investments and insisted the PM was in the vanguard of efforts to increase the transparency of tax arrangements.
But he was accused by opposition parties of failing to follow through with promises to force reform in UK crown dependencies and overseas territories which act as tax havens and faced calls for a full independent investigation.
More than 11 million documents were passed to German newspaper Suddeutsche Zeitung and shared by the International Consortium of Investigative Journalists (ICIJ) to 107 media organisations including the Guardian and BBC's Panorama.
HM Revenue and Customs has approached the ICIJ for access to the data and said it would "act on it swiftly and appropriately" if there was any wrongdoing.
While there is nothing illegal about using offshore companies, the disclosures have intensified calls for international reform of the way tax havens are able to operate and claims of large-scale money laundering.
Mr Cameron has been a vocal advocate of reform and legislation forcing British companies to disclose who owns and benefits from their activities which comes into force in June.
Despite several years of pressure however, few UK Crown Dependencies and Overseas Territories - which are said to make up a large part of the tax havens referred to in the papers - have taken concrete action to open up the books.
He faces pressure to secure progress at an international summit on tackling corruption which he will chair in London in May and where the use of offshore tax havens to escape scrutiny will be high on the agenda.
Asked if Mr Cameron was prepared to legislate if there was continued inaction, the PM's official spokeswoman said: "He rules nothing out. The work with them continues."
Foreign Secretary Philip Hammond, who recently met the Panamanian vice president to discuss the issue, insisted "significant progress" was being made.
"It's always interesting when information like this leaks because it reminds people who are up to no good how fragile and how vulnerable they make themselves by indulging in this kind of activity," he told the BBC.
"This is a key agenda for the Prime Minister."
Shadow chancellor John McDonnell said not enough had been achieved.
"Cameron promised and has failed to end tax secrecy and crack down on 'morally unacceptable' offshore schemes," he said.
"Real action is now needed."
Liberal Democrat leader Tim Farron called for an independent investigation in light of the allegations, claiming the UK Government has "failed to act until forced to by a scandal" over tax havens.
He said: "Rather than waiting until more headlines force action, the Government should now commission a full independent investigation into the way the Virgin Islands and others have acted, what the UK Government knew and why they have not used their legislative powers to impose the transparency rules they previously claimed to support."
Among the reported disclosures are a suspected 2 billion dollar (£1.4 billion) money laundering ring run by a Russian bank and said to involve close associates of President Vladimir Putin.
The Kremlin denied the president was connected to any wrongdoing and claimed he was the target of a smear operation.
Others who have been caught up in the disclosures include Icelandic Prime Minister Sigmundur Gunnlaugsson who is facing calls for his resignation over claims that he had an undeclared interest in his country's bailed-out banks.
In China, the Guardian said, the families of at least eight current and former members of the supreme ruling politburo had been found to have hidden wealth offshore.
And 23 individuals who had had sanctions imposed on them for supporting the regimes in North Korea, Zimbabwe, Russia, Iran and Syria were said to have been revealed to have been clients of Mossack Fonseca.
Ian Cameron's use of the firm to help shield investments from UK tax as he built up a significant legacy, part of which was inherited by the Prime Minister, had been previously disclosed but further adds to the pressure on the PM.
There is no suggestion that this avoidance arrangement or others exposed by the leak were anything but entirely legal or that Mr Cameron's family did not pay the UK tax due on any repatriated assets.
Oxfam head of UK policy Richard Pyle said: "People in the world's poorest countries pay the highest price for the billions of lost tax money when their governments are unable to fund life-saving healthcare.
"The UK is in a unique position to help clean up the murky world of tax havens - starting by ensuring that the real beneficiaries of shell companies registered in the UK's Crown Dependencies and overseas territories, such as the British Virgin Islands, are revealed ahead of May's anti-corruption summit in London."
Mossack Fonseca said it had operated "beyond reproach" for 40 years and had never been charged with criminal wrong-doing.
SNP deputy leader Stewart Hosie said: "David Cameron must act now to crack down on tax evasion and aggressive tax avoidance - as he has previously promised to do - starting with immediate transparency of company beneficiaries.
"The wealthiest in society should pay their fair share and should not be able to avoid paying tax on huge parts of their earnings, whilst those on middle and low incomes work hard to pay their taxes in full.
"Currently, true ownership of companies registered in British Overseas Territories is shrouded in secrecy due to the use of nominated directors and shareholders making it impossible for tax authorities to find out who really owns and benefits from the company. This has to stop and the Tories are running out of excuses as to why they have not done more to stop this dubious practice."