David Cameron will meet world leaders next month to work on new tax avoidance measures but today's massive leak of the Panama papers could overshadow the London summit.

No 10 today insisted the Prime Minister and Britain had been global leaders on cracking down on tax evasion and aggressive avoidance.

But after the leak of millions of papers - suggesting among others Mr Cameron's father and a string of Tory donors sheltered money off shore - threatens to undermine the talks.

Campaigners have demanded British legislation to end UK tax havens - including in the crown dependencies of Jersey, Guernsey and the Isle of Man.

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David Cameron, right at a summit in Washington with Barack Obama last week, struck a deal on international tax avoidance with the US President and other G8 leaders in 2013. He is hosting a follow up meeting in London next month

The Prime Minister placed tackling global tax avoidance at the heart of his G8 presidency in 2013 and is working to follow up on the agreements made.

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.

Mr Cameron's official spokeswoman today insisted good progress had been made since 2013.

She said: 'We want to see the overseas territories and crown dependencies play their part and that is why we will continue to push them to do so and as I have said the Prime Minister has made clear that should they fail to do so, he rules absolutely nothing out.'

WHAT ARE OFFSHORE ACCOUNTS AND HOW ARE THEY USED? WHAT ARE OFFSHORE OR SHELL ACCOUNTS? Offshore bank accounts and other financial dealings in another country can be used to evade regulatory oversight or tax obligations. Often, companies or individuals use shell companies, initially incorporated without significant assets or operations, to disguise ownership or other information about the funds involved. WHERE ARE MOST OFFSHORE ACCOUNTS? Panama, the Cayman Islands and Bermuda are among more than a dozen small, low-tax locations that specialize in handling business services and investments of non-resident companies. LEGITIMATE USES FOR OFFSHORE ACCOUNTS: Companies or trusts can be set up in offshore locations for legitimate uses such as business finance, mergers and acquisitions and estate or tax planning, according to the global money laundering watchdog, the Financial Action Task Force. ILLICIT USES OF OFFSHORE ACCOUNTS: Shell companies and other entities can be misused by terrorists and others involved in international and financial crimes to conceal sources of funds and ownership. The ICIJ says the files from Mossack Fonseca include information on 214,488 offshore entities linked to 14,153 clients in 200 countries and territories. EFFORTS TO CRACK DOWN ON FINANCIAL HAVENS: Financial and legal professionals get training on how to spot potential violations, since in some cases lawyers and bankers are unaware they are handling illicit transactions. The EU has stepped up efforts to crack down on tax avoidance by multinational corporations. Advertisement

Asked if any cash should be repaid she added: 'That is a matter for HMRC.'

Mr Cameron's spokeswoman said Britain was 'leading the pack internationally' on the issue and said 90 countries were now signed up to policies agreed at the G8 in 2013.

Turning to next month's summit, she added: 'We will be looking at a range of areas - this was something the Prime Minister decided last summer ahead of the G7 meeting in Germany that we should try focus on this issue as a country that is meeting its commitment to 0.7 per cent aid spending.

'We want to look now at how we move forward to make sure in different walks of life money is being spent in the right way and people know where taxpayers money is being spent.

'May's summit will be an opportunity to bring together leaders, international organisations, NGOs, to look at is there more we can do in this area to do with tax and transparency or, indeed, in other areas such as sport.'

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.'

SNP Treasury spokesman Stewart Hosie today said the Government had to go further on British Overseas Territories.

He said: '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.'

Ahead of the 2013 summit, Mr Cameron insisted tax havens had a right to be low tax jurisdictions but said rules had to be transparent and enforced fairly.

From June, firms registered in Britain have a legal obligation to obtain and hold adequate, accurate and current information on the ultimate owner who benefits from the company.

The biggest financial data leak in history today revealed how Vladimir Putin's inner circle and a 'dirty dozen' list of world leaders are using offshore tax havens to hide their wealth.

A host of celebrities, sports stars, British politicians and the global rich are all implicated in the so-called Panama Papers - a leak of 11million files which contain more data than the amount stolen by former CIA contractor Edward Snowden in 2013.

Documents were leaked from one of the world's most secretive companies, Panamanian law firm Mossack Fonseca, and show how the company has allegedly helped clients launder money, dodge sanctions and evade tax.

Megastars Jackie Chan and Lionel Messi are among the big names accused of using a controversial Panama law firm to invest their millions offshore.