Online publisher Real Buzz, which runs the official UK website for the New York marathon, has introduced package deals to boost participation in other major marathons around the world, including those in Boston, Berlin and Chicago.
Other tour operators offer places in the New York marathon, but one charity, which asked not to be named, told Third Sector that because Real Buzz was the official internet partner for the race it was the operator most charities and runners were likely to use.
A source at the charity said: "We bought a package that included places for Chicago, even though we didn't want them. Real Buzz has effectively got a monopoly." The charity in question has reduced its sponsorship targets to take into account the increased expenses that have to be met by runners.
Another charity, which also asked not to be named, said it had a good relationship with Real Buzz, but the cost of taking part in marathons was becoming prohibitive. "With time, only bigger charities will be able to afford to take part," a source said.
Tim Rogers, chief executive of Real Buzz, said: "There are other operators offering places in the New York marathon. We do not have a monopoly."