The official count of Venezuelans who participated in the election of an all-powerful constituent assembly was tampered with - off by at least one million votes - the head of a voting technology company said.
The finding is certain to sow further discord over the super-body that has generated months of nationwide protests.
Smartmatic CEO Antonio Mugica said results recorded by his systems and those reported by Venezuela's National Electoral Council indicate "without any doubt" that the official turnout figure of more than eight million voters was manipulated.
The international software company has provided electronic machines in Venezuela since 2004.
"Even in moments of deep political conflict and division we have been satisfied with the voting process and the count has been completely accurate," Mr Mugica told reporters in London.
"It is, therefore, with the deepest regret that we have to report that the turnout figures on Sunday, 30 July, for the constituent assembly in Venezuela were tampered with."
Mr Mugica did not specify if his company's figures showed one million more - or one million fewer - voters participated in the election.
The assembly will be granted vast powers to rewrite the nation's constitution and override every branch of the government.
Opposition members boycotted the election, claiming the terms to select delegates were heavily rigged to favour the ruling party.
President Nicolas Maduro (pictured below) has vowed to use the assembly to target his enemies and solidify Venezuela as a socialist state.
Mr Maduro says the body will help end the violence and protests that have engulfed the country since March, with at least 125 reported dead and some 2,000 wounded.
Despite the unrest and plummeting popularity ratings, Mr Maduro appears to have maintained the full support of the country's most important institutions, notably the armed forces.
Top military figures have been given special status and are scattered throughout the government.
They also are in charge of strategic areas such as food distribution in which Venezuelans say bribery is widespread.
An independent exit poll concluded Sunday's turnout was less than half that reported by the election commission.
Opposition leaders who stationed observers in every municipality also questioned the count.
Julio Borges, the president of the opposition-controlled National Assembly, said politicians will ask the nation's chief prosecutor to investigate commission members for potential crimes.
"They want to give total power to an assembly who people with more than 10 years managing the electoral process in Venezuela say with all certainty, irreversibly, with hard data from their own servers, that the announced result was crooked," Mr Borges said.
The assembly is expected to be sworn into office on Thursday.
Opposition leaders are calling on Venezuelans to gather in Caracas for a mass protest.
Despite widespread international condemnation, Mr Maduro has appeared undeterred in his plans to seat the constituent assembly this week.
He has threatened to use the assembly's virtually unlimited powers to go after his opponents - and the jailing of two prominent opposition leaders on Tuesday appeared to show he is willing to proceed with full force.
Leopoldo Lopez and Antonio Ledezma were dragged from their homes by heavily armed security agents and thrown into a military prison.
The nation's Supreme Court is accusing them violating the terms of their house arrest by plotting to escape and releasing video statements criticising Mr Maduro.
Both men's supporters denied the charges and vowed to continue to try to push the ruling socialist party from power.
However, they gave little indication of how they planned to do that, and the capital was unusually quiet after months of sometimes violent protests.