MADRID, Nov 23 (Reuters) - Spain's Socialist government accused the previous administration of backing a short-lived coup against Venezuelan President Hugo Chavez in 2002, a charge the conservative opposition labelled a "lie".

The shock accusation by Spanish Foreign Minister Miguel Angel Moratinos threatened to boil over into an embarrassing political spat during a visit by Chavez to Madrid to patch up relations damaged by the coup.

"Under the previous government, in an unprecedented move for Spanish diplomacy, the Spanish ambassador (in Caracas) received instructions to support the coup," Moratinos told state television in an interview broadcast late on Monday.

Moratinos said Spain's Socialist government, which took office in April, was focused on improving relations with Latin American governments, especially those with strong democratic support such as Venezuela's.

"This will never be repeated, we respect the popular will," said the career diplomat.

Chavez, who was briefly ousted in a coup in April 2002, was dramatically restored by loyal supporters and troops after being held captive for two days by rebel generals and admirals.

A populist former paratrooper who led a failed 1992 military uprising, Chavez accused the previous Spanish government of Jose Maria Aznar of backing the uprising against him.

Moratinos' comments, however, were the first Spanish support for his accusations.

"I have no doubt that that is what happened. The Spanish ambassador was the only one, together with the U.S. ambassador, who recognised the tyrant -- a tyrant put in place via a blood-bath and a break with the institutional norm," Chavez said in an interview on Spanish state TV on Tuesday.

However, a spokesman for Aznar's Popular Party denied the allegations.

"The foreign minister lied yesterday and slandered (former) Prime Minister Aznar," party spokesman Eduardo Zaplana told a press conference, adding his party would react strongly.

Zapatero's surprise election victory in March, only three days after a major bombing in Madrid, ushered in an abrupt improvement in relations with Caracas.

While Aznar's government was one of the strongest supporters of the U.S.-led war on Iraq, Zapatero swiftly pulled Spanish troops from the coalition.

Spain is an important investor in energy-rich Venezuela, with oil company Repsol set to invest more than 450 million euros there between 2004 and 2008, according to media.