Cardinal Renato Martino, one of Benedict's top aides, was scheduled to fly to London to attend the bond sale on behalf of the German pontiff .
"In this way, Benedict XVI wants to show his full support for this initiative," said the Italian prelate, who heads the Vatican's 'Justice and Peace' department .
Britain is one of eight countries backing a new international financing initiative which aims to speed up the flow of cash for vaccinations against killer diseases such as polio, malaria and tuberculosis .
The others are France, Italy, Norway, Spain and Sweden, who have already committed funds, and Brazil and South Africa who have promised to do so soon .
The International Financing Facility for Immunisation, or IFFI, works by getting governments who have pledged funds for immunisation aid in the future to issue bonds now .
The money lent by bond purchasers goes straight into the vaccination programmes in some 72 of the world's poorest countries. This means important campaigns are financed immediately .
"The sum of the bonds purchased will go directly to the most needy populations and in particular to the large scale vaccination of under-fives," Martino said in a statement .
An anticipated IFFI investment of 4 billion dollars is expected to help prevent five million child deaths between 2006 and 2015 .
In addition, by protecting more than 500 million children against measles, tetanus and yellow fever, it is hoped that more than five million adult deaths will be avoided in the future. The IFFI initiative, which is backed by the World Bank, was designed to help rich countries achieve some of the Millennium Development Goals agreed by the UN in 2000 .
Among those goals, which were given a 2015 deadline, is reducing child mortality and combatting malaria and other diseases for which vaccinations exist .
Britain's Chancellor of the Exchequer Gordon Brown first presented his plan to issue an IFFI bond during a conference on poverty and globalisation held in the Vatican in 2004 .
Britain has pledged the most funds - 1.38 billion pounds over 20 years - to the vaccination programmes organised in connection with IFFI. Italy, another important donor, has pledged, 473 million euros .