BUCHAREST (Reuters) - Romania's Constitutional Court approved a law on Monday making it easier to impeach the president, but said any referendum must have at least a minimum turnout to be valid, a ruling which could yet derail a government attempt to remove Traian Basescu.

The dispute between leftist Prime Minister Victor Ponta and right-wing President Basescu has raised international concerns over respect for the law in the European Union's second-poorest country.

Parliament last week approved a measure by Ponta's Social Liberal Union (USL) to suspend Basescu saying he had overstepped his powers. An impeachment referendum will take place on July 29.

The USL passed a law which requires only a majority of those voting in a referendum to impeach the president, rather than a majority of the whole electorate. The court ruled the law constitutional.

However, the court also said that turnout must be more than 50 percent of the electorate for the referendum to be valid, which could yet give the unpopular Basescu a chance.

Turnout at last month's nationwide local elections, which the USL dominated, was about 56 percent. But if Basescu's supporters do not vote, turnout could fall below half.

"The law ... over the organization and protocol of the referendum is constitutional, provided that participation in the referendum is at least half plus one of the number of people registered in permanent electoral lists," it said in a statement.

The political chaos has raised doubts over Romania's International Monetary Fund (IMF) aid deal, sent the leu plunging and pushed borrowing costs higher. The currency regained some ground on Tuesday after the court ruled Basescu's suspension was constitutional, but it is still close to lows.

"It is important to note that the EU may find Romanian steps against its laws and it can halt the IMF/EU bailout program," KBC analysts said in a note.

(Additional reporting by Radu Marinas, Sam Cage and Luiza Ilie; Editing by Robin Pomeroy)