Turnout figures in Venezuela's Constitutional Assembly election were manipulated up by least 1 million votes, Smartmatic, a company which has worked with Venezuela since 2004 on its voting system, said on Wednesday.

"We know, without any doubt, that the turnout of the recent election for a National Constituent Assembly was manipulated," Smartmatic CEO Antonio Mugica said at a news briefing in London.

Mugica said Smartmatic, which has provided electronic voting technology for elections around the world, was able to detect the overstated officially announced turnout because of Venezuela's automated election system.

"We estimate the difference between the actual participation and the one announced by authorities is at least 1 million votes," he said.

Mugica declined to directly answer whether the manipulated turnout numbers changed the result of the election, in which authorities said 8.1 million people voted.

The election of the legislative super-body has been decried by critics as illegitimate and designed to give the unpopular government of President Nicolas Maduro powers to rewrite the constitution and sideline the opposition-led congress.

Mugica said the authorities in Venezuela would likely not be sympathetic to his comments and that he had not yet passed the evidence to the Venezuela's electoral council.

The National Elections Council did not immediately respond to a request for comment.

According to internal electoral council data previously reviewed by Reuters, only 3.7 million people had voted by 5:30 p.m. in Venezuela's election on Sunday.

Venezuelan authorities did not respond to a request for comment on that reviewed data.