LONDON, March 13 (Reuters) - Developing countries miss out on tax receipts worth more than the billions of dollars in foreign aid they receive because their own nationals put cash in offshore tax havens, British charity Oxfam said on Friday.

Developing countries lose as much as $124 billion in taxes a year, more than their yearly $103 billion in foreign aid, a study for the charity showed.

People from developing countries hold more than $6.2 trillion abroad and capital flight is increasing by $200-300 billion per year, according to the study for Oxfam by a former chief economist at management consultancy McKinsey & Co.

Reform of tax havens would be an easy win for our leaders that would benefit ordinary people at home and abroad alike, Kristy Hughes, Oxfam's head of policy, said in a statement, noting that $16 billion would provide each child a school place.

Combating offshore tax evasion is on the agenda for the April 2 G20 group of developed and emerging countries in London.

The global financial crisis has seen cash-strapped Western governments taking tougher steps against tax evaders and the crisis also looks set to crimp development aid. (Reporting by Petra Spescha and Catherine Bosley; Editing by Louise Ireland)