Rare stamps, including sheets registering original designs from the reigns of George V – himself a fanatical stamp collector, who paid a then world record price of £1,450 for a Mauritian stamp in 1904 – Edward VIII and George VI, are to be auctioned by the British Postal Museum and Archive. The sale is expected to raise more than £5m to help fund a new purpose-built museum, certain to become a place of pilgrimage to stamp collectors.

The Sotheby's auction in July will include registration sheets, the first stamps printed from an approved new printing plate, particularly coveted by collectors, which are among the greatest rarities of the stamp world.

Only two copies of the registration sheets were ever produced for the stamps in the sale, including the "seahorse" stamps from the reign of George V which are regarded as masterpieces of engraving. The archive is retaining one of each original sheet, which will never be sold, but putting the only other copies into the auction.

Richard Ashton, a philately consultant to Sotheby's, described the stamps in the sale as of the utmost rarity and beauty.

"Now in my 50th year as a professional philatelist, I have never seen such an important sale of its type. Many are the only examples of their kind ever to come on the market," he said.

Most of the vast postal museum collection and archive, covering 400 years of postal history including mail coaches and historic red pillar boxes, and every British stamp issued since the Penny Black – described by mayor of London, Boris Johnson, as "a national treasure of global importance", is in store. The new museum is due to open in 2016 at Mount Pleasant in central London.