Arts minister Baroness Blackstone has blocked the export of one of the most expensive paintings to be sold in the UK in a last effort to save it for the nation.

British institutions now have to raise about £12.5m to keep the work, Portrait Of Omai.

The picture by the 18th Century artist, Sir Joshua Reynolds, depicts the first South Sea islander to visit Britain.

Until its sale in November last year, the painting had formed part of the Castle Howard collection, in North Yorkshire, for more than 200 years.

Tate Britain had wanted to buy the painting when it came up for sale last year, but its offer was rejected.

The portrait eventually sold for £10.3m, the second highest price for a painting in this country - The Lock by Constable set the record when it was sold in 1990 for £10.7m.

But a committee of experts which looks into art exports has ruled that every possible effort should be made to raise funds to keep Portrait of Omai in the UK.

Baroness Blackstone has stepped in to place a temporary bar on the painting leaving the UK.

This will initially last for three months and could be extended if serious efforts are made to save the work.

At present, the portrait is held at the stores of auctioneer Christies at Vauxhall, south London.