Liberian election officials have acknowledged there is an error on ballot papers for Tuesday's constitutional referendum.

Voters were supposed to choose between two retirement ages for Supreme Court justices: 70 or 75. But the ballot papers list the two choices as 75 and 75.

Election commission officials blame the situation on a printing error. They say the error was noticed only a few days ago, too late to make a change.

Officials say voters are being given instructions on how to vote, despite the mistake.

Liberians are casting votes on four proposed changes to the constitution. The most controversial change would reduce the number of years a presidential candidate must have lived in Liberia, from 10 years to five.

The amendment, if passed, could affect which candidates are eligible to run in the presidential election later this year.

Liberia's main opposition party has called on voters to boycott the one-day referendum.

Other proposed changes would delay the presidential and parliamentary elections from October to November and eliminate run-off votes in legislative polls.

Two-thirds of voters must approve the changes for them to be adopted.

The referendum is widely seen as a test of whether Liberia is ready to hold the coming elections.

The country is still recovering from 14 years of civil unrest and war that ended in 2003. President Ellen Johnson Sirleaf was elected president in the country's first post-war polls in 2005.