The Governor of the US state of Texas - Greg Abbott, has ordered that voters can drop off their mail-in ballots at only one location per county in the lead-up to the presidential election.

Abbott, a Republican, said the move would stop illegal voting. In a statement following the order on Thursday, Abbott cited Covid-19 in calling for strengthening ballot-security protocols, the BBC reported.

"These enhanced security protocols will ensure greater transparency and will help stop attempts at illegal voting."

Numerous nationwide and state-level studies over the years have revealed no evidence of major, widespread fraud in postal voting.

Critics have accused Abbott of voter suppression, pointing out that in some cases this means thousands of voters must go to one clerk's office.

Coming weeks before the election, the order requires counties to close any satellite ballot drop-off locations.

Texas is the second most populous US state and it would be a big win for candidates during the November 3 election. The state has been reliably Republican in modern presidential contests.

Many more voters this time around are expected to vote by post or drop off their ballots due to the pandemic.

Mail-in voting protocols differ slightly state by state, but in Texas, eligible early voters can begin dropping off their ballots this month.

Some of the state's largest counties had already set up multiple drop-off sites. Harris County, home to the city of Houston and some four million residents, must now close 11 drop-off locations, the Houston Chronicle reported.

In addition to raising questions about how potentially millions of urban Texans will need to visit a single drop-off site, the move may also be problematic for rural residents, who are spread out across the state - which is also America's second largest by area.

The order goes into effect on Friday.

Texas Democratic chairman Gilberto Hinojosa responded to the announcement by accusing "scared" Republicans of trying to change rules while "on the verge of losing".

Hinojosa added that courts across the US have said it is too late to make changes to election rules, "but our failed Republican leadership will try anyway".

The proclamation by Abbott will cause "widespread confusion and voter suppression", said Harris County clerk Chris Hollins in a statement. Harris County, which encompasses Houston, is the most populous county in the state.

"Multiple drop-off locations have been advertised for weeks," he said. "To force hundreds of thousands of seniors and voters with disabilities to use a single drop-off location... is prejudicial and dangerous."

Abbott's order also says voting officials must allow poll watchers to be present at the drop-off site.