The Archers was first exhibited at the Royal Academy in 1770

The masterpiece, currently the subject of a export ban, is one of the 18th Century artist's most important pieces of work.

It is the centrepiece of a forthcoming exhibition of his work at Tate Britain.

The Tate has until July to raise the extra �3.2m needed to save the work from being sold, possibly abroad.

Market value

Tate director Sir Nicholas Serota said: "Today's announcement of the Art Fund's major grant towards our fundraising goal will enormously help our efforts to secure the painting for the national collection."

The full-length portrait - Colonel Acland And Lord Sydney: The Archers - was painted in 1769 and first exhibited at the Royal Academy in 1770.

The Tate has until 26 July to match the agreed market price for the painting, but it has also secured a pledge of �500,000 from Tate members.

Former Arts Minister Estelle Morris placed the export ban on the painting after the reviewing committee on the export of works of art awarded it a starred rating, meaning every effort should be made to raise money to keep it in the UK.

Historical significance

The Tate has also been involved in a long-running battle to buy another Sir Joshua painting, the Portrait Of Omai.

The Tate Gallery unsuccessfully tried to buy the picture from its anonymous owner for �12.5m after a ban was issued preventing the painting from leaving the UK.

The 18th Century painting has remained in storage but the owner has agreed to allow it to be part of an exhibition.

Portrait of Omai fetched the second highest amount for a British painting when it was sold at auction for �10.3m in 2001.

It was bought by a London dealer who sold it on to a collector, who wanted to take it out of the country, but was barred from doing so by the government because of its historical significance.