By Steve Kingstone BBC News, Sao Paulo

Jefferson and Lula are members of the same ruling coalition

Pressure is mounting in Brazil for a detailed parliamentary inquiry into alleged bribery involving the governing Workers' Party.

According to the allegations, the party paid bribes to guarantee the ongoing support of its coalition partners.

The Workers' Party initially issued a statement expressing surprise.

But later it emerged that President Luiz Inacio Lula da Silva had personally discussed the issue with the politician making the accusations.

The timing could not be more unfortunate.

Just as the government welcomes delegates to the capital, Brasilia, for the Fourth International Forum on Fighting Corruption, the party led by the president stands accused of offering bribes in return for parliamentary support.

As yet, there is no concrete evidence to support that central claim, which was made by Roberto Jefferson, the president of the Brazilian Labour Party, a member of the ruling coalition.

But the government's response to the allegation has been inconsistent - and, arguably, misleading.

Dwindling popularity

At first, the Workers' Party said there was absolutely no truth to the claims.

[The allegations] haven't got the slightest bit of truth Jose Genoino Workers' Party president Brazil stocks down on probe

But later, it emerged that at least part of Mr Jefferson's story was true.

In March, he had met President Lula in person to discuss the corruption claims.

But a subsequent parliamentary investigation was shelved for lack of evidence.

Now, opposition lawmakers want a more detailed inquiry.

Through his meeting with Mr Jefferson, the president is at this stage personally tied to the outcome.

Mr Lula's supporters argue that Mr Jefferson is a desperate man, already facing his own bribery allegations.

But opinion polls suggest that public trust in the government is slipping - as is investor confidence.