Voters across the UK are today able to cast their ballots in elections to the European parliament.

For the purposes of the election the UK is divided up into 12 constituencies, each of which will return multiple MEPs to the Brussels parliament.

The distribution of seats is as follows:

• Eastern: 7
• East Midlands: 6
• London: 9
• North East: 3
• North West: 9
• South East: 10
• South West: 7
• West Midlands: 7
• Yorkshire and the Humber: 6
• Scotland: 7
• Wales: 4
• Northern Ireland: 3

Under the proportional representation system in operation, voters can cast one vote for a party or independent candidate.

Each political party prepares a list of candidates ranked in order to match the number of seats to be filled in that region.

Once votes are counted, the first seat is allocated to the party or independent candidate with the highest number of votes. If an independent candidate is highest, the seat is allocated to that individual. If a party wins, the seat is taken by the person on top of that party's regional list.

After the first allocation, a winning party's total number of votes is divided by two and the second seat is allocated to the next party or independent candidate with the highest number of votes.

The process then continues until all seats have been allocated.

To ensure that voting across Europe is completed before any results are announced, counting will not begin until Sunday evening.

Labour

While part of the campaign had centred on calls for the poll to be turned into a referendum on the Iraq war, Jack Straw sought to turn the debate back to the benefits of EU membership.

The foreign secretary has urged voters to recognise that the elections will help to determine "how our country pursues its interests in the European Union".

In a speech last week he argued that Britain's "full and active membership of the European Union is a vital part of our future prosperity and security"

He also warned against "taking for granted" benefits such as unrestricted rights to travel, work and study across the EU.

He accepted the EU is "far from perfect" and called for further reforms.

"But let's be clear: withdrawal from the European Union would be an unmitigated disaster for Britain," he said.

"It would threaten British jobs, British businesses and British influence in the world.  It would put our future prosperity and security at risk."

Conservatives

In its campaigning, the Conservative Party has focused on its "put Britain first" theme.

In a keynote campaign speech, the Tory leader warned voters they face a clear choice in the elections.

"At one extreme there are the candidates from the UK Independence Party," Michael Howard said.

"They represent a party that wants to pull out of the European Union altogether. They have frequently failed to vote in the European parliament on issues that are vital to Britain

"At the other extreme are Labour and the Liberal Democrats - who want to transfer even more from Britain to Brussels, setting Europe on the path to a single European state."

He has vowed that the Conservatives will press for a "flexible Europe" in which Britain is not forced to agree to the continuing centralisation of power.

"We should look at taking back powers from Europe that would be better exercised at a national level here in Britain - and in other countries too," he said.

Lib Dems

Liberal Democrat leader Charles Kennedy kicked off his party's European election push with a call for voters to punish the government for its stance on the Iraq war.

It was a theme Kennedy returned to later in the campaign when he argued that because of its anti-war stance his party was attracting increasing support from Britain's Muslim community.

But the party is also highlighting its constructive approach to European issues.

"We don't scaremonger about Europe. But neither are we supine in our attitude towards Europe," says Kennedy.

"Of course the EU isn't perfect, neither is Westminster. But reform can only come from within.

"That is why Liberal Democrats are committed to improving the EU, attacking waste and inefficiency, reforming the unfair common agricultural and common fisheries policies."

UKIP

With the high profile support of TV star Robert Kilroy-Silk and an endorsement from Hollywood's Joan Collins, the UK Independence Party has made much of the running in the election campaign.

It has also been boosted by the support of some Conservative peers and some opinion polls suggesting the party could push the Lib Dems into fourth place today.

UKIP leader Roger Knapman says his party is "strengthening by the day, and even by the hour".

He adds that the UKIP is "taking increasing numbers of people from all the political parties, and from the UK’s biggest party, the 'stay at home' party".

Greens

Meanwhile, the Green Party has accused the UKIP of "whingeing from the sidelines while doing nothing about EU reforms".

"UKIP always do their best to stir up voters' concerns over waste and inefficiency in the European parliament," said Dr Spencer Fitz-Gibbon, a member of the Green Party's national executive.

"But they don't seem to be interested in reform. There's a campaign to end much of the wastefulness in the EU, like the €200 million spent every year maintaining a parliament in both Brussels and Strasbourg, yet not one UKIP MEP has supported this reform.

"They are evidently happy to occupy seats under false pretences, because they don't think British MEPs should even be there.

"But when it comes to the practicalities of making the EU more efficient, they're just not interested."

Nationalists

SNP leader John Swinney has conducted a "whistlestop tour" across Scotland during the campaign.

The nationalist party has been highlighting the fact that many of the new EU member states are smaller than an independent Scotland would be.

And fishing has also been a key theme, with warnings that the SNP will vote no to the European constitution unless the common fisheries policy is scrapped.

Elsewhere on the campaign trail, Plaid Cymru has also been warning that EU enlargement could see Wales losing out.

The current five Welsh MEPs will be reduced to four from June 10.

"Our aspirations are for Wales to be able to enjoy the benefits which Ireland has gained from Europe but, in practice, we lag behind even the new entry states," says nationalist MEP Jill Evans.

"Latvia's population is 500,000 less than Wales yet, as a new member state, it will have more than double the number of MEPs which we will have."

Northern Ireland

With Rev Ian Paisley stepping down after 25 years' service in the European parliament, the DUP is pinning its hopes on Jim Allister.

"The DUP succeeds in Europe because we come with a clear and unambiguous message of putting Ulster first," says Allister.

Sinn Fein's Gerry Adams meanwhile has predicted his party will make "considerable advances".

They are also campaigning in the Republic of Ireland where they hope to seize at least one seat.

"We're bringing in every single vote, we are asking people to vote for Sinn Fein and to transfer to Sinn Fein," he said.

For the Ulster Unionist Party, Jim Nicholson is standing on his record of service in Brussels, while highlighting opposition to the new European constitution and euro membership.

The SDLP's Martin Morgan has set out his commitment to "build on John Hume's legacy of delivery for all the people of the North and beyond".

"I will be part of the large and influential Socialist Group in the parliament, which will help enormously in keeping the North on the EU agenda," he says.

Click here for more information on the candidates who are standing for election.