Polls have opened in the UK for thousands of Australians to vote on whether the Queen should remain as their head of state.

The first ballot was cast by the country's High Commissioner in the UK, Philip Flood, who registered his vote at 1000BST.

Australians who wanted to be among the first of their fellow countrymen to add their vote to the vital consitutional debate, had formed a queue outside the High Commission in The Strand, London.

Voting rules allow those Australians living abroad to vote early - the rest of the country will go to the polls on 6 November.

The polling station at Australia House is expected to receive votes from more than 20,000 Australian expatriates in the next two weeks - either by post, or in person.

It will be the largest polling station in the referendum on whether the country should become a republic and appoint its own head of state to replace Queen Elizabeth II.

Subdued debate

Mr Flood said: "Australians always take voting obligations very seriously and we believe it is important for our citizens to have every opportunity to cast their vote, even those 12,000 miles away from home."

The High Commissioner and his wife Carole declined to disclose how they voted in the secret ballot.

Opinion polls in Australia indicate a close contest between monarchists and republicans which indicates the ballot in Britain could prove critical.

Republicans require a double majority to change the country's constitution. As well as a majority of the national vote, they also need majorities in four out of the six states.

Experts are predicting vote to be close

Voting in the referendum is compulsory, but debate on the subject has been remarkably subdued and Prime Minister John Howard says most Australians are not interested.

Just two months ago, 50% of voters were unaware a referendum was to be held. Of those who did, many said they did not know enough to vote one way or the other.

Opinion polls say Australians will vote overwhelmingly to cut their ties with Britain and the Queen.

'Deals behind closed doors'

But opposition has arisen to the proposal that the republic's head of state, the President, will be chosen by members of the Australian Parliament.

Critics say it could result in political deals being struck behind closed doors.

Voting in London will be between 1000BST and 1700BST, but opening hours will be extended until 2000BST on 28 and 29 October and on 4 and 5 November.

Polling on Saturday, 30 October will be between 1000BST and 1400BST.

There will be no voting on 31 October or 6 November.