A tougher crackdown on tax evasion by the world's wealthiest individuals could provide cash-strapped G20 governments with up to $100bn (£62bn) in much-needed tax revenue, the Organisation for Economic Cooperation and Development said on Thursday night.

The Paris-based thinktank told the leaders of developed and developing countries that they could fill black holes in their public finances and improve social cohesion by co-operating to remove tax loopholes and by exchanging information.

A survey of 20 rich and poor countries conducted by the OECD showed that earlier measures to deter tax evasion had resulted in 100,000 individuals paying a total of $14bn in unpaid tax on assets worth between $120-150bn.

Jeffrey Owens, director of the OECD's centre for tax policy and administration, said: "That is just the tip of the iceberg. There is probably $1tn in assets held offshore."

The call came as all G20 governments agreed on Thursday to a multilateral convention to tackle tax evasion more effectively.

Until now, only seven of the G20 — including Britain — had signed up to the agreement to provide mutual administrative assistance in tax matters, which includes automatic exchange of information on request, multilateral simultaneous tax audits, and international help in the collection of tax.

The agreement also imposes safeguards to protect confidentiality of information exchanged.

G20 action to combat clamp down on tax havens began at the London summit in April 2009, but some campaign groups have argued that the agreement reached was too weak, with evasion remaining rampant.

Owens said the London G20 had resulted in 700 separate agreements being signed, with countries changing legislation to allow access to previously secret information, including that relating to banks. But there had been a problem with tax havens for 50 years and the problem was not going to be solved overnight.

"Now that the G20 countries have led by example, we expect other countries to sign the convention," he added. "As the membership expands, so the effectiveness of the convention will increase. Over the coming months we will be working with developing countries so that they will rapidly be in a position to sign the convention."

Owens said there were three reasons why governments were becoming keener on tax co-operation. "Fiscal consolidation means that governments need money," he said. "It also shows the taxpayer that everybody is paying their fair share, not just the little guy."

Owens said the final reason governments were toughening up their approach was that they wanted to raise more revenue from multi-national companies involved in sophisticated tax planning.

Angel Gurría, the OECD's secretary general, said: "Today we have taken a major step forward to improve global tax cooperation. The OECD looks forward to continuing to work with the G20 and other countries to maximize the benefits from this powerful multilateral instrument. Tax co-operation and compliance are of crucial importance for all countries and citizens – and not only in times of a tight fiscal and budgetary environment."

The thinktank said the G20 accord reinforced international co-operation to target tax evasion by both individuals and corporations.