Nearly 40% of all the subsidiary and associate companies of the UK's 100 biggest publicly-listed businesses are registered in tax havens, according to research by a charity.
ActionAid - which fights against poverty around the world - said despite a promised government crackdown on tax havens and scandals over tax avoidance, the number of FTSE 100 companies with offshore havens has not dropped.
It called upon David Cameron to tackle the issue at the G8 summit in June, and said the world's poorest countries were suffering because their wealth was being "siphoned" away.
The charity's analysis of the FTSE 100's subsidiaries, joint ventures and associated companies were compared to similar research released 18 months ago.
It found the number of FTSE 100 multinational groups with companies in tax havens - 98 - is the same as it was in October 2011, and ten of them have their headquarters in tax havens - up from nine in 2011.
The study revealed 78 of the FTSE 100 do business in developing countries, and every new entrant to the FTSE 100 since 2011 has tax haven companies.
Nearly 9,000 of the FTSE 100's 22,000 overseas companies are located in tax havens, and while the total number of overseas companies decreased since 2011, the proportion of those located in tax havens rose from 37.8% to 38.2%.
ActionAid said tax havens were allowing multinational companies and wealthy individuals to drain billions of pounds from poor countries.
Mike Lewis, the charity's tax justice policy adviser said: "Tax havens are one of the biggest hidden obstacles in the fight against global poverty.
"Poor countries lose an estimated three times more money to tax havens than they receive in aid each year - money needed to build roads, fund schools and finance developing countries' own fight against hunger and poverty.
"Four years after G20 leaders promised an end to tax havens, tax haven structures are near-universal amongst the UK's biggest multinationals.
"Now, with David Cameron promising action on tax havens at this year's G8, the problem is on the UK's doorstep. The UK is responsible for 1 in 5 of the world's tax havens - that's more than any other country."
The ActionAid researchers found 58% of FTSE 100 banks' overseas companies were in tax havens - 1,780 - and almost a third of the countries in which FTSE 100 banks operated were developing countries, yet they have 13 times as many companies located in tax havens as in developing countries.
Mike Lewis added: "Stopping wealth being siphoned out of the poorest countries into tax havens is one of the most urgent tasks in the fight against hunger. But transparency deals that the UK and other European countries have recently struck with tax havens currently benefit rich countries only.
"David Cameron convenes the G8 leaders in Northern Ireland next month. They must deliver on their promise to call time on tax havens for the benefit of all countries, including the poorest."
ActionAid said while it is not accusing FTSE 100 companies of tax avoidance or evasion, its findings highlight the extent of multinationals' operations in jurisdictions that can provide substantial tax advantages and help obscure information.
Catherine McKinnell MP, Labour's shadow exchequer secretary to the Treasury, said: "These figures show the scale of the challenge we face in tackling tax avoidance.
"But the government has failed to bring forward the measures Labour has called for to increase transparency and end the era of tax secrecy.
"And they have also failed to deliver the international leadership we need to secure wider reforms to the corporate tax system. It's time David Cameron and George Osborne acted."