The acting speaker of the lower house of Brazil's Congress has annulled last month's vote to impeach President Dilma Rousseff.
Waldir Maranhao's request means the matter will go back to the Chamber of Deputies and could delay the process by days or weeks.
Mr Maranhao said the process was tainted by irregularities that ended up invalidating the April 17 vote in the lower house. The decision grew out of allegations by solicitor general Jose Eduardo Cardozo contending that political parties should not have recommended their members on whether or not to vote in favour of the impeachment.
The announcement has sparked debate over the move's legality and possible implications.
Opposition leaders have threatened to appeal the decision to the supreme court, and observers say the government could do the same if the senate decides to ignore the request and hold the proceedings anyway.
Ms Rousseff is battling impeachment over allegations her government violated fiscal rules, in what critics say was a bid to artificially hold up the country's flagging economy. Ms Rousseff said previous presidents have used such fiscal manoeuvres and the impeachment effort amounts to a "coup" aimed at removing her and her left-leaning Workers' Party, which has governed the country for 13 years.
Ms Rousseff has reacted cautiously to the news saying: "We have a difficult fight ahead of us."
She also called for caution, adding "we live in a time of cunning and wile".
Mr Maranhao took over the reins in the Chamber of Deputies, after former Speaker Eduardo Cunha - Ms Rousseff's sworn nemesis and the driving force behind the impeachment effort - was removed from office over corruption and obstruction of justice allegations against him. Mr Maranhao voted against the impeachment in the April 17 vote in the Chamber of Deputies.
Opposition congressman Pauderney Avelino said from his point of view, the impeachment process was out of the lower house's control.
"The process in the chamber is done with," Mr Avelino said.
The head of the Brazilian Order of Attorneys, Claudio Lamachia, said: "This sort of action responds to the momentary interests of certain political groups but ignores legitimate decisions that have already been made.
"Brazil is in a political intensive care unit, at the peak of an ethical and institutional crisis."
The impeachment proceedings come as Brazil is grappling with the biggest recession in decades, an ongoing corruption probe that has ensnared top politicians and prominent businessmen and also an outbreak of the Zika virus. The country's capital, Rio de Janeiro, is gearing up to host the Olympics in August.
Ms Rousseff's approval ratings have dipped into the single digits in recent months. While polls have suggested broad public support for her impeachment, they have also pointed to widespread trepidation about who might replace her.
Demonstrations favouring impeachment have been called for in cities including Rio and Brazil's economic capital, Sao Paulo.