Former Brazilian President Luiz Inacio Lula da Silva and his wife, Marisa Leticia, have been summoned to testify to an enquiry into corruption at the state oil company, Petrobras.
A prosecutor is investigating whether the former president failed to declare ownership of a seafront penthouse flat.
It now belongs to construction firm OAS, which has been implicated in the Petrobras corruption scandal.
“The suspicions are unfounded,” Lula’s foundation said in a statement.
“The accusations over the alleged hiding of assets by former President Lula and his family are frivolous,” added the statement.
His lawyers rejected the allegations of money laundering.
They said Mr Lula’s wife was given the option to buy the flat but they never purchased it.
The luxury penthouse in the resort of Guaruja is estimated to be worth up to $550,000, Brazilian media reported.
Sao Paulo state prosecutor Cassio Conserino told Globo television that there are indications that the former president tried to hide the fact that owned the flat.
Mr Lula will testify in Sao Paulo on 17 February.
A former president of OAS, Leo Pinheiro, will also be heard on the same day.
He was sentenced to 16 years for corruption but has been freed after appealing against his conviction.
Prosecutors estimate that the corruption scandal at Petrobras cost the state-owned company $2bn (£1.4bn).
Several politicians, businessmen and Petrobras officials have been convicted.
Prosecutors say the country’s top building companies paid bribes to senior officials in the company to secure lucrative, overpriced contracts.
More than 40 top politicians – including the presidents of both houses of Congress – have also been under investigation.
Mr Lula’s former chief of staff Jose Dirceu and the Workers Party treasurer Joao Vaccari are among those arrested in connection with the scheme.
The scandal has damaged the popularity of President Dilma Rousseff, who succeeded Mr Lula in 2011.
Ms Rousseff is not implicated in the corruption scheme, but she was head of Petrobras during the years when much of the corruption is believed to have taken place.
Opinion polls show that her approval rates have plummeted since she was re-elected in October 2014.