In thaw of Iran ties, Britain's Hammond to reopen ransacked embassy in Tehran

* 'Old Fox' Britain to reopen embassy to Iran

* Western relations with Iran in major thaw

* Iran to reopen embassy in London - Hammond

* First visit by British Foreign Secretary since 2003

By Guy Faulconbridge

KUWAIT, Aug 23 (Reuters) - In a signal of the most striking thaw in the Western ties with Iran for over a decade, Foreign Secretary Philip Hammond will reopen the British embassy in Tehran nearly four years since protesters ransacked the elegant ambassadorial residence and burned the Union Jack.

The nuclear deal that the Islamic Republic struck with six major world powers last month has prompted a flurry of European visits - including from German and French ministers - aimed at positioning for the end of Iran's long economic isolation.

But Britain has operated without an embassy since Iranian protesters stormed its two main diplomatic compounds in Tehran on Nov. 29, 2011. The protesters slashed portraits of British monarchs, torched a car and stole electronic equipment.

Following the storming which Prime Minister David Cameron called a 'disgrace', Britain shut the embassy and expelled Iran's diplomats from London. Iran will simultaneously re-open its embassy in London on Sunday, Hammond said in a statement

"Our relationship has improved since 2011," said Hammond, who will be only the second British foreign minister to visit Iran since the 1979 Islamic revolution that toppled the U.S.-backed shah. The last visit was by Jack Straw in 2003.

"Four years on from an attack on the British Embassy, I am today re-opening it," Hammond said.

The statement confirms an earlier report by Reuters on Thursday.

The Tehran and London embassies will initially be run by chargé d'affaires but ambassadors will be agreed with months, Hammond said.

Accompanying Hammond is a small group of business leaders, including representatives from Royal Dutch Shell, Energy and mining services company Amec Foster Wheeler and Scottish industrial engineering firm Weir Group.

"RETURN OF THE FOX"?

After more than a decade of casting the Islamic Republic as a rogue power seeking to sow turmoil through the Middle East, Britain has sought to improve ties with Iran, whose proven natural gas reserves are as vast as Russia's.

"In the first instance, we will want to ensure that the nuclear agreement is a success, including by encouraging trade and investment once sanctions are lifted," Hammond said.

Under the nuclear deal, sanctions imposed by the United States, European Union and United Nations will be lifted in exchange for Iran agreeing long-term curbs on a nuclear programme that the West thought was intended to make a nuclear bomb.