It could be a bad heir day for Royal Mail. The postal giant has admitted it may lose the right to use symbols of the monarchy including the E II R royal cypher on post boxes and the crown on stamps after its £3bn privatisation, while Prince Charles' image may never appear on future stamps.

Details set out deep inside Royal Mail's 447-page prospectus for potential investors reveal Royal Mail is party to an agreement with the Government, signed on 27 September, the day the privatisation was launched, covering its use of royal symbols on stamps.

Whilst the Queen's head will remain on stamps as long as she reigns, under the Postal Services Act other royal symbols are not guaranteed. The Lord Chamberlain's office has told the postal giant that ministers may ban it from using Crown trademarks in the event of a string of different circumstances.

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The Lord Chamberlain's letter also states that the image of the next monarch is not guaranteed to grace future stamps. It will be up to Prince Charles to decide whether to let them use his face. Other events could also see Royal Mail stamps stripped of royal insignia, including the firm ceasing to provide a universal postal service or using the royal associations in a way which "brings the Crown into disrepute." The Queen can also withdraw her consent to use of the royal cypher at any time.

What's in a name? Why our post is anonymous

Britain is the only country whose postage stamps do not feature the name of the country issuing them – because we were the first country to produce stamps.

It was 6 May, 1840 when Great Britain issued the world's first adhesive postage stamp, with the Penny Black showing Queen Victoria.

From then on all British stamps were issued with a portrait - often in profile or semi-profile - of the reigning sovereign. Kings, unlike queens, never wore crowns in their stamp portraits.

Stamps marking special events were introduced in 1924, for the British Empire Exhibition at Wembley. Now around 12 sets of special commemorative stamps are issued every year.