The government's liability for public sector pensions has grown so much in recent years that it now outstrips the national debt.

Actuarial consultants estimate that outstanding pensions payments total £690 billion.

The revelation comes as trade unions are today staging nationwide protests against the changes ministers are planning to make to the pensions of public sector workers.

The protests will be an unwelcome distraction for Labour, coming amid a tough general election campaign where the party is already concerned about a lack of grassroots support.

Senior union figures have expressed their frustration with the plans being put forward by the government.

And thousands of people are expected to attend protests in towns and cities across the country.