British general election: What the polls say

PIC: Main opposition Labour Party leader Jeremy Corbyn delivers a speech at the County Hotel in Carlisle, Cumbria, on June 4, 2017 in response to the terror attack on London Bridge. (AFP)

With just a few days to go until Britain's general election, opinion polls reveal the outcome could be a lot tighter than had been predicted when Prime Minister Theresa May announced the vote six weeks ago.

Although surveys show the gap between the main two political parties narrowing, May's position as prime minister seems secure.

What the polls say

May surprised the country in April by calling for the snap election, seeking to increase her majority before Britain enters into two years of gruelling negotiations over its departure from the European Union.

Polls initially supported her gamble, giving her Conservative Party a double-digit lead over its nearest rival, the main opposition Labour Party.

However, the Conservatives' advantage has eroded over the campaign, with pollster Survation giving the ruling party just a one point lead over Labour on June 4.

Another poll, released a few days earlier by YouGov, even suggested the Conservatives could fall short of a majority, meaning they would need the support of another party to govern.

Can the polls be trusted?

Pollsters got the outcome of the last general election, held just two years ago, very wrong.

In the months leading up to the May 2015 ballot, polls consistently put the Conservatives and Labour neck-and-neck, suggesting neither party would be able to form a government alone.

But the Conservatives, who had been in a coalition government with the smaller Liberal Democrat Party, secured a majority in the 650-seat House of Commons.

A year later, polls also failed to correctly predict the outcome of the Brexit referendum, expecting the "In" vote to win.

"One of the things that happened in 2015 is that the polls underestimated the age difference in turnout," John Curtice, professor of politics at Strathclyde University, told AFP.

That underestimated Conservative support among older voters.

Pollsters have therefore adjusted their methodologies by widening their pools of respondents, asking them more questions and weighing the result with high-quality academic research done since the last election, explained Curtice.

Why are predictions so tough?

Part of the reason is Britain's electoral system.

"There is no automatic relationship between votes cast at the national level and seats won," because of the first-past-the-post constituency system, Curtice said.

The system makes it especially difficult for smaller parties with support evenly spread nationally to increase their share of seats in parliament.

But smaller parties whose support is concentrated in key constituencies -- such as the Scottish Nationalist Party -- can do very well.

What will new government look like?

According to Curtice, "there doesn't seem to be any prospect of the Labour Party winning this election".

But the Conservative government could end up looking very different with pollsters predicting that the party could gain anything between one and 80 seats.

"That's the difference between a prime minister returning with more latitude to achieve her objectives and a prime minister returning a wounded animal," said Curtice.