The work is called The Kongouro from New Holland, or The Kangaroo, and the arts minister Ed Vaizey on Tuesday announced an export bar on it and a painting of a dingo by Stubbs.

Vaizey was acting on the recommendation of a committee which monitors the export of important works of art. He deferred a decision on an export licence until at least August.

The two paintings would have offered Britons the first look at the strange creatures from the new world that were being talked about as a result of Captain Cook's expeditions to the Pacific in the late 18th century.

They are likely to be sold to a foreign buyer unless a UK buyer can raise a matching offer of £5.5m. Lord Inglewood, chairman of the Reviewing Committee on the Export of Works of Art and Objects of Cultural Interest, said: "It would be a terrible shame if the UK were to lose these extraordinary paintings to an overseas buyer. They were the British public's first introduction to these exotic animals from the Australasian new world which was opening up at that time."