Voters in Texas are having their selection flipped to an entirely different party by the state’s 20-year-old ballot machines, officials have admitted.

Dozens of people attempting a straight-ticket choice – where they vote for the same party in all posts being contested – have reported their commands being disobeyed by the devices.

Republican voters say they have found themselves staring at screens telling them they have selected democrat Beto O'Rourke for the US Senate. Democrats say the same machines have told them they have favoured the Republican Ted Cruz for the same position.

It is unclear if anyone has actually voted for the wrong party as a result of the flipping as the machines give users the chance to correct any errors before confirming.

But the issue has added one more controversy to one of the most acrimonious midterm elections in American history.

Concerns with the devices were first raised on social media but state officials confirmed on Friday they were aware of the problem – which they then blamed on voter misuse.

Beto O'Rourke and Ted Cruz debate for the Texas US Senate: Tom Reel/San Antonio Express-News via AP, Pool

“The Hart eSlate machines are not malfunctioning,” said Sam Taylor, spokesman for the office of Secretary of State, according to the Press Association news agency. “The problems being reported are a result of user error – usually voters hitting a button or using the selection wheel before the screen is finished rendering.”

The Hart eSlate machines in question are used in 30 per cent of Texas counties, including the state’s largest, Harris, which includes the city of Houston.

In a statement on Friday, Ted Cruz said he had heard of “multiple reports” of race selections changing and told supporters: “Once you select the Republican party ticket, please be patient and do not select ‘next’ until the ballot has populated all of the selections.”

Gilberto Hinojosa, the Texas Democrat party chairman, demanded a public service announcement to warn voters, training for poll workers on the issue and the removal of “malfunctioning machines”.

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But Zenén Pérez, of the Texas Civil Rights Project, said the problem would keep happening until the state increased investment in its election infrastructure.

"Texas just hasn't spent the resources necessary to modernize," he told GQ magazine. "Until they do, we're going to continue to see this kind of thing on a regular basis."