South Africans living abroad should be allowed to vote, according to a ruling by Pretoria High Court.
The decision could mean a delay to elections, due in April or May, to allow time for the law to be changed.
The court found current laws breached the rights of South Africans living abroad and referred the judgment to the Constitutional Court for confirmation.
The government says it will not comment until it has studied the ruling. Some two million South Africans live abroad.
John Battersby, of the Global South Africans Network in the UK, said that of these, 5-600,000 were in the UK.
"It might be a difficult decision to implement in a short space of time," he told the BBC's Focus on Africa programme.
The court ordered the electoral commission to change its procedures to let South Africans living abroad to vote.
The opposition Afrikaner nationalist Freedom Front Plus party had brought the legal application on behalf of a South African school teacher living in the UK.
Speaking outside the court, party spokesman Willie Spies told Sapa news agency it was still possible the Constitutional Court, the country's highest court, could overrule the judgment.
The party said it was filing an application for the proclamation of election day to be postponed to allow the Constitutional Court ample time to consider the matter.
The date for the elections was expected this week.
Currently certain groups, including government employees and people on holiday and business trips, are entitled to vote while out of the country.
The poll is shaping up to be the most interesting since the end of apartheid in 1994.
African National Congress (ANC) leader Jacob Zuma is the front-runner to become president but his bid has been overshadowed by the corruption charges he is facing.
Bitter divisions in the ruling party recently led to the launch of the breakaway Congress of the People (Cope) party, which could win enough votes to deny the ANC a two-thirds majority to parliament, needed to change the constitution.
The tensions have spilled over into clashes between groups of rival supporters.