The rally, which will feature talks from CWU general secretary Billy Hayes along with MPs, takes place ahead of the expected unveiling of a bill for part-privatisation to Parliament this Thursday.

According to The Independent, Royal Mail staff have been warned that their pensions could be significantly cut unless the mooted privatisation plans took place.

In a letter to Business Secretary Lord Mandelson, Jane Newell, chairman of the Royal Mail's pension trustees, said there would be "devastating consequences" as a result of such a plan, as well as a larger deficit in the pension fund than the original £5.9bn figure.

However, supporters of the proposed sale, which is believed to be a stake of around 30%, say the move would go some way in helping to fund a modernisation of Royal Mail's service.

Lord Mandelson has previously said that a healthy Royal Mail is "vital to sustaining the universal service".

He added that partial privatisation of Royal Mail would give the business "the confidence, the experience and the capital to make the changes needed to improve performance and face the future".

Alex Walsh, head of postal affairs at the DMA, has previously put his weight behind the proposals having said the association "would like to see an efficient, profitable Royal Mail and it has been questioned before whether this can happen in its current structure".

In a previous poll by the CWU, 75% of those questioned opposed plans to privatise part of Royal Mail, a figure that rose to 89% when possible foreign ownership was suggested.