BRASILIA (Reuters) - Prosecutors investigating Brazil's biggest-ever graft scandal threatened to resign en masse on Wednesday if a move to gut an anti-corruption bill won approval from legislators as the nation mourns an air disaster.

The lower chamber of Congress passed the bill in the early hours of Wednesday morning by 450 votes to 1, with changes that would help shield lawmakers from prosecution and weaken the authority of public prosecutors.

The vote came as Brazil grieves for soccer club Chapecoense following an air crash on Monday night in which 71 people died, including all but three of the team's players and several journalists.

"In the dead of night, they took advantage of a moment of national mourning and shock to subvert the proposals," said Deltan Dallagnol, leader of the team of investigators probing a massive political kick-back scheme centered on state-run oil company Petrobras.

Dallagnol accused the lower chamber of seeking to block Operation Car Wash as it comes close to incriminating a "significant number" of lawmakers in the Petrobras scandal.

The anti-corruption bill originated in a petition signed by 2.5 million Brazilians frustrated at widespread graft.

Prosecutors on the taskforce called a news conference to denounce the changes to water down the bill.

"We plan to resign collectively if this proposal is signed into law by President Michel Temer," prosecutor Carlos Fernando Lima said.

As Brazilians mourned the victims of the Colombian crash, lawmakers removed the legal definition of the crime of illegal enrichment and scratched a clause creating a reward and protection system for informants of corruption.

Instead, they added penalties, including prison sentences, for abuses of authority committed by judges and prosecutors.

Brazil's top prosecutor Rodrigo Janot said in a statement the changes were clearly aimed at "intimidating and weakening" the authority of prosecutors and the judiciary.

The overwhelming support for the bill reflected concern over an impending plea bargain deal with the country's largest engineering conglomerate Odebrecht, in which executives are expected to inform on bribes paid to as many as 200 politicians in recent years.

The bill still needs Senate approval. Critics of the changes are hoping Temer will veto the measures if it clears the Senate.

Presidential spokesman Marcio de Freitas said Temer would only decide when and if it came to his desk. "The anti-corruption measures are still in Congress. We must wait to see what gets approved," he told Reuters.

(Reporting by Anthony Boadle; Editing by James Dalgleish)