Postal workers will start a fresh wave of industrial action tonight, which will disrupt mail deliveries for the next two weeks.

The Communication Workers Union (CWU) said the aim was to hit Royal Mail hard, at minimal cost to its members.

Individual workers will take part in two 24-hour walkouts, but the action is being staggered over the next fortnight, with different groups of staff taking action on different days.

The union is also aiming to step up its political campaigning and to explain to MPs why it has rejected a 2.5% pay offer and the Royal Mail's modernisation plans, which the CWU warned will lead to the loss of 40,000 jobs.

The postal organisation said it was "hugely disappointed" that the union was pressing ahead with more strikes.

A fresh row broke out last night over proposals to tackle a £6 billion shortfall in the Royal Mail pension fund.

The union said the Royal Mail had printed 170,000 copies of a document outlining proposed changes to pensions, which officials said suggested managers had already decided to close the final salary pension scheme to new staff from next year and raise the age at which workers can retire with a full pension from 60 to 65 by 2010.

But the Royal Mail insisted it had decided some weeks ago not to publish or proceed with the document.