Books would cost less to post but bulky items such as posters would be more expensive under plans put forward today by the Royal Mail.

The organisation said it wanted to introduce a "fairer and simpler" pricing system based on size instead of weight.

The proposals would spark the biggest change in postal pricing since the Penny Black and could be "disastrous" in industries such as film processing, warned the postal watchdog.

But the Royal Mail said the price for sending three quarters of mail would not change.

Large lightweight and bulky items would cost more to send, but heavier, compact goods would be cheaper to post.

Under the plans there would be five stamp prices for first and second class mail instead of the current 29.

David Dale, a Royal Mail spokesman, said: "The current pricing structure has been in place for decades and is inherited from a time when all mail was hand-sorted, and so is increasingly out of date.

"Most other postal administrations already use size in pricing mail, because this is the key cost factor in the modern postal industry.

"These changes would not increase the nation's total post bill. While most customers wouldn't see any changes, some would have price cuts and others rises."

Postcomm, the industry regulator, said that the proposals would involve major change, although would not produce extra revenue for the Royal Mail.

Nigel Stapleton, the Postcomm chairman, said: "They mean that light but large mail such as very large greetings cards, CDs, rolls of photographic film, audio cassettes and video tapes, would cost more to post, but that some heavier items - books for example - would cost less.

Postwatch, the consumer watchdog, said that the changes could make it cheaper to send brochures and company reports, but warned that it could be "disastrous" for receiving photographs after they have been processed.

Responses to the plans have to be made by the end of July.

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