A prayer book as beautiful as a piece of jewellery and once owned by King François I of France in 1532 has been temporarily barred from export by the government to give a British institution the chance to raise the £8m price paid by an overseas collector.

The gold-enamelled cover, studded with rubies, turquoise and tourmaline, and the 20 beautiful illustrations painted on parchment, make the book one of the most expensive ever. It has been in UK collections since 1720, and was once owned by Horace Walpole, a collector of works of art with royal connections, as one of the greatest treasures of his eccentric Strawberry Hill folly castle home in south-west London.

The book, made by a workshop specialising in luxury books for aristocratic patrons, was bought by the French king, who was renowned for his support of the arts. He was a patron of Leonardo da Vinci, and his collection became the basis of the Louvre museum. It later passed through several aristocratic owners, including the king’s sister Marguerite d’Angoulême, Henry IV, and Cardinal Mazarin, surviving centuries of political and religious tumult in near perfect condition.

The independent committee which advises on export licence applications described it as of “exceptional splendour and of outstanding significance”. The export application has been deferred until October and could be extended if any institution has a realistic chance of raising the money.