Britain plans to spend an extra 600 million pounds on so-called free schools outside municipal control over the next three years, the country's Finance Ministry said on Saturday.

Free schools are the flagship education policy of the Conservative-Liberal Democrat coalition that came to power in May 2010, and can set their own curriculum and staff employment conditions, unlike most other state-funded schools.

Twenty-four of the schools -- based on models in Sweden and the United States -- opened in September.

The Treasury said the new money would fund an extra 100 schools before the next national election in May 2012.

Teaching unions opposed to the plans say the schools are too few in number, and too geared towards middle-class families, to tackle the low attainment widespread among children from poor backgrounds.

Some of the money will go to a new type of free school aimed at teaching high-level mathematics to 16- to 18-year-olds.

A Treasury spokeswoman said that Chancellor George Osborne would explain where the funding for the schools will come from in his autumn budget statement on Tuesday.

Osborne faces an uphill struggle to convince Britons that he has clear plans to boost the economy at a time when growth is rapidly slowing and the government has committed to hefty cuts in overall public spending.

In the past week, the government has announced a 1-billion- pound package to aid young unemployed people, and a 400-million-pound scheme to promote house building.

(Reporting by David Milliken; Editing by Jan Paschal)