Voting districts in the US state of Florida must replace most of their touchscreen electronic voting machines with optical scan machines, according to the terms of a bill that governor Charlie Crist has signed into law.

The bill, sponsored by Crist, was passed by the state legislature earlier this month.

At the time, Crist said he filed the bill in response to experts that said touch-screen systems, also known as DRE (Direct Recording Electronic) machines, were vulnerable to hackers, were unreliable and made voters doubt their ballots were counted.

Optical scan systems require that paper ballots be filled out by voters for tabulation, which leaves a paper trail of votes that can be used in case a recount is required. DRE systems do not generally offer paper-based records of votes.

The bill does allow precincts to use touch-screen systems only for handicapped voters that require its features. Federal law requires that machines be available to allow handicapped persons to vote unaided.

"There is no greater testament to our nation's democracy than the people's ability to choose their leaders," Crist said in a statement after signing the bill. "When Floridians cast ballots in an election at any level - local, state or federal - they can leave the polling place knowing that their vote has been counted and recorded and can be verified," he stated.

The bill also provides $27.8m to the Department of State for use in purchasing optical scan equipment.