A Puerto Rican teachers' association has filed a lawsuit against the government of the US territory following approval of changes to the teachers' retirement system.

The lawsuit filed on Wednesday asks that the court finds the law, which switches pension provision from a defined benefit plan to a defined contribution system, unconstitutional.

Governor Alejandro Garcia Padilla claimed that the changes were necessary because the system has a $10 billion (£6bn) deficit and said it would run out of money by 2020 if nothing was done.

However, his motives have more to do with threats by rate-setting agencies to downgrade Puerto Rican bond to junk status if public expenditure remains high.

The public school teachers have called a 48-hour strike starting on January 14, when classes resume.

The Broad Front in Defence of the Retirement System for Teachers, which unites different unions and associations across the education sector, has organised the strike to force the governor to back down.

And Puerto Rico Teachers Association chief Aida Diaz has asked all teachers and parents to support the initiative.

The teachers plan to spend January 14 and 15 presenting the Labour Department with an alternative to the government's cost-cutting retirement system.

The teachers complain that the amendments include a substantial worsening of their retirement conditions.

Thousands of teachers with 30 years of service are likely to retire in May to avoid a substantial cut in their pensions, Educamos union leader Eva Ayala has warned.

Nearly 8,000 of Puerto Rico's roughly 36,000 teachers will be eligible to retire at the end of the 2013-2014 academic year.

Those who decide to leave in May will receive an annual pension equal to 75 per cent of their final salary.

But teachers who delay their retirement beyond May 2014 will get pensions equal to only 65 per cent of their final salaries.

The reform also increases the employee contribution from 9 per cent to 10 per cent and establishes a minimum retirement age of 62 for new teachers.

Puerto Rico's teachers are not integrated into the US social security system.