Development agencies are urging the Chancellor to take the lead in ending tax dodging at a meeting of G7 finance ministers this week.
The IF campaign said UK tax havens were the "elephant in the room" at the meeting, which gets underway on Friday.
Campaigners warn that developing countries are losing out on vital income that could be used to ensure their people have enough food to eat.
They are disappointed that the UK's tax havens are not on the agenda at this week's meeting, saying it should be an opportunity to fulfil David Cameron's commitment at Davos to use this year's G8 to clamp down on tax dodging in poor countries.
With one in five of the world's tax havens under UK jurisdiction, IF campaigners are asking the Chancellor to lead the way by implementing a 90-day plan to clampdown on the UK's own tax havens.
People have been tweeting George Osborne throughout the day to show their support for the IF campaign's call.
To highlight the need for change, a pop-up boardroom was set up next to the Treasury in central London, complete with a large inflatable elephant and a campaigner dressed as George Osborne.
Matthew Frost, chief executive of IF member organisation Tearfund said: "As Christians, we believe that we must handle money with integrity.
"Tax avoidance is a scourge on our world, and millions of people suffer as a result of the greed and selfishness of some very rich people and companies.
"We hope and pray that world leaders will this year recognise the importance of effective tax systems and close the loopholes that mean people and companies can either avoid paying tax or that governments can withhold information from their citizens about how they use taxpayers' money."
The UK last week announced a new information sharing deal with a handful of tax havens but IF campaigners say this will benefit only the UK, France and Germany, Italy and Spain, while poor countries were not party to the agreement.
Figures released by IF ahead of the G7 meeting show that Cayman Islanders are 320,000 times wealthier than those living in Nicaragua.
While assets on the Cayman Islands total $29.3 million for each of the 56,000 people on the island, this compares to just $91 in assets for every Nicaraguan living just 600km away.
Similarly, in Bermuda assets amount to $1.42m for each of its 65,000 residents, while in nearby Haiti this is just $18 per person.
IF campaign spokesperson, Sol Oyuela said: "These figures expose the global system of tax havens as a sick joke. Who in their right mind would claim the Cayman Islander should be worth 320,000 times than a Nicaraguan?
"It can't be right that UK tax havens are sitting on trillions of dollars, while their neighbours are struggling to ensure their people have enough to eat.
"The money that UK-linked tax havens suck out of the poorest economies will be the elephant in the room when finance ministers meet."