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Pope Innocent X presided over the influential Vatican state in the seventeenth century but the real power behind the papal throne belonged to the Pope’s sister in law and reputed lover Donna Olimpia Maidalchini.

A feminist before her time, she was immortalised by one of the greatest ever portrait painters, Diego Velázquez.

The painting has been missing for 300 years, until now, and it's going on sale at Sotheby's in London with a price tag of £3 million.

Culture Correspondent Stephen Smith reports.