An impeachment referendum held on 29 July yielded an 88% majority in favour of removing the centre-right Băsescu, but the turnout fell short of the 50% required - only 46% of registered voters cast a ballot during the referendum.
Ponta's Social Liberal Union (USL) had pressured the Constitutional Court to rule that the electoral lists had not been updated and that the turnout requirement had been met. The judges ruled, by a 6-3 margin, that the referendum was invalid.
Asked if Băsescu should now return to office, Chief Judge Augustin Zegrean said "Yes".
Ponta , whose USL controls the parliament, said he would respect the court ruling.
But the cohabitation between the centre-right president and USL could yield further tensions ahead of the November parliamentary election, which is likely to be won by the USL coalition. Băsescu has the power to appoint prime ministers.
The country needs to focus on austerity policies to keep a €5-billion International Monetary Fund stand-by agreement on track, Reuters reported.
Analysts said the battle between Ponta and Băsescu reflects a wider struggle for power and the justice system in the country hobbled by corruption and 19 members of the USL parliamentary coalition are under investigation.
The centre-right European People’s Party in the European Parliament called on the Romanian authorities to respect the Constitutional Court’s decision.
“The Romanian people need responsible political leadership that focuses on the reform and development of their country and not on the personal political agendas of socialist and liberal leaders,” stated Joseph Daul, leader of the EPP group.
Crin Antonescu, who heads the Romanian liberal party PNL and is part of the governing coalition, held the post of interim president pending the decision of the Constitutional Court, and should now resign.
But Antonescu made statements that sounded at the least confusing.
“For us Traian Băsescu is no longer a president. He is the president of Romania in the legal sense. But the decision of the Constitutional Court has been unfair,” Antonescu said.