May won't get a majority: Antony Green

British Prime Minister Theresa May's conservative party will be returned but fall short of a majority, election analyst Antony Green says.

But she will be judged for making the "wrong decision" to call the snap June 8 election, Mr Green has told the ABC.

With 356 seats counted, the conservatives have 43.1 per cent of the vote share and Labor 39.7 per cent, the analyst tweeted after midday on Friday (AEST).

"Jeremy Corbyn has gained seats for Labour. He's not going to be the next Prime Minister," Mr Green earlier told ABC TV.

"What they (people) will say is that 50 days ago, the government called an election expecting to get an increased majority and they've gone backwards.

"The question will be over whether that was the correct decision, and I think given the results ... the judgement will be that the wrong decision was made."

Mrs May called the snap 8 June election in April, three years early.

Polls at the time suggested a conservative lead, and Mrs May wanted to increase her parliamentary majority and strengthen her Brexit negotiating position.

The BBC has forecast the Tories are forecast to take 322 seats in the 650-member parliament.