Stamps are valid until election of new pope.

The Vatican has issued a special set of four "Sede Vacante" stamps to mark the period between the death of Pope Francis and the election of his successor.

The stamps, which are seen as highly collectible, are only issued during the interregnum period when the Holy See is vacant before the election of a new pope.

The Sede Vacante went on sale in Vatican post offices on Monday and will remain valid for postal correspondence mailed from Vatican City until the next pope is elected.

However the post offices in St Peter’s Square will continue to sell the stamps to collectors after the election of the new pope, for philatelic use.

The series of stamps comes in four values - €1.25, €1.30, €2.45 and €3.20 - with the words: Sede Vacante MMXXV and Città del Vaticano.

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The stamps depict angels holding up two symbolic keys, one gold and one silver, surmounted by a canopy or ombrellino in place of the papal tiara.

This is the heraldic arms of the Vatican camerlengo who acts as the administrator of the Holy See during the Sede Vacante, with the role being performed by Cardinal Kevin Farrell.

In addition, a special collector's folder containing the series of Sede Vacante stamps will go on sale in Vatican post offices on Friday.

This is the eighth series of Sede Vacante stamps to be issued since the Vatican City was established as an independent state in 1929; the first series was released in February 1939 following the death of Pope Pius XI.

The last Sede Vacante stamps went on sale in March 2013 following the retirement of Pope Benedict XVI.

Work is currently underway to transform the Sistine Chapel ahead of the conclave when cardinals will gather to elect a new pope, a secretive process scheduled to begin on Wednesday.