Since 31 July 1635 when King Charles I issued a proclamation extending the use of the Royal Mail to the public, the British postal service has based its charges on the weight of the items delivered.

As of April 2006 this will change to a new size-based postal tariff - a move the RM claims will result in fairer pricing as it is the size and shape of letters and packages that impact on handling and sorting costs.

Assuming the move is approved by postal regulator Postcomm, it will have a major impact both on the UK's direct mail industry and direct-to-buyer distribution channels such as the internet and mail order.

Says a RM spokeswoman: "Basically, this is about getting our prices to reflect our costs. Heavy but compact post will be cheaper to send. We have had to go through the costs with the regulator. As far as our profits are concerned, it is a revenue-neutral move."

She claimed that around eighty per cent of residential mail will be unaffected by the changes. As to the remaining 20%, the impact of the new pricing will be 50/50 - around half of senders paying more, half less.

The RM move emulates similar pricing systems in the US, Japan and Australia. It will not affect the postal service's obligation to deliver mail at a uniform price throughout the UK - even to the most remote offshore island addresses.

Effective 1 January 2006, the Royal Mail's 370-year letter delivery monopoly will end, in line with the European Union's liberalisation of member states' postal services.

Data sourced from BBC Online; additional content by WARC staff