Britain's Conservatives have lost their overall majority in a snap general election that has resulted in a hung parliament.
With just a handful of seats left to declare, Thursday polls shows gains for the opposition Labour Party.
This is seen as a humiliation for Prime Minister Theresa May, who chose to call the election to try to strengthen her hand in talks with the EU on Brexit.
Labour leader Jeremy Corbyn urged her to resign, but she said her party would "ensure" stability in the UK.
However, Mrs May believes she has the numbers to form a government and will visit the Queen to seek authorisation to do so at midnight (NZ time), the BBC's Laura Kuenssberg says.
And Sky News are reporting that Northern Ireland's Democratic Unionist Party (DUP) will give the Conservative Party their backing.
DUP said there was no need for a formal coalition deal in order to back Conservatives.
"At this time more than anything else, this country needs a period of stability," Mrs May said earlier.
"And if, as the indications have shown and if this is correct that the Conservative Party has won the most seats and probably the most votes, then it will be incumbent on us to ensure we have that period of stability - and that is exactly what we will do."
Mr Corbyn earlier said: "If there is a message from tonight's results, it's this: the prime minister called this election because she wanted a mandate. Well, the mandate she's got is lost Conservative seats, lost votes, lost support and lost confidence."
"I would have thought that's enough to go, and make way for a government that will be truly representative of all of the people of this country," he added.
The pound earlier fell sharply in value after the BBC/ITV/Sky Exit poll was published when the voting ended at 10pm BST (9am NZT).
Final election results are expected by Friday lunchtime (late evening NZT).
The biggest shock of the night so far has been Liberal Democrat MP Nick Clegg losing his seat to a Labour candidate. He was deputy prime minister of the UK from 2010 to 2015 in a coalition government with the Conservatives.
Former Scottish National Party leader Alex Salmond was also defeated, losing his seat to a Tory contender.
The UKIP leader Paul Nuttall has stepped down, saying there must be a new era for the party after its vote collapsed.
A total of 650 Westminster MPs will be elected, with about 45.8 million people entitled to vote. A party needs 326 seats to have an overall majority.
Mrs May - who had a small majority in the previous parliament - called an early election to try to improve her negotiation positions on Brexit.
But analysts say it now appears the prime minister made a serious miscalculation.
The BBC's political editor, Laura Kuenssberg, says Mrs May's decision may prove to have been one of the biggest political mistakes of modern times.
The latest forecast suggests that the Conservatives are 10 seats short of an overall majority.
Labour are predicted to gain 33 seats, the Tories lose 15 seats, the Lib Dems will gain five and the SNP lose 22 seats.
The Green Party would be unchanged with one seat and Plaid Cymru still have three MPs in Wales, according to the poll.
Northern Ireland has different political parties.
Reaction: 'Disaster for Theresa May' - SNP
SNP leader Nicola Sturgeon said the election "has been a disaster for Theresa May".
"Her position I think is very, very difficult. We have to wait and see how things shake out," Mrs Sturgeon said, adding that she was "disappointed at the SNP losses".
UK Independence Party leader Paul Nuttall tweeted: "If the exit poll is true then Theresa May has put Brexit in jeopardy. I said at the start this election was wrong. Hubris."
Lib Dem president Baroness Brinton said her party could not work with either Labour or the Tories as both are pushing for a "hard Brexit".
Green co-leader Caroline Lucas earlier said she could "hardly dare hope" that the exit poll was right, adding: "To be clear, Greens will never support a Tory government."
Why the election matters - it's all about Brexit
The election will largely determine the UK's negotiation policies in upcoming negotiations with the EU on Brexit.
Mrs May was against Brexit before last year's referendum - but now says there can be no turning back and that "Brexit means Brexit".
The reason the prime minister gave for calling the election was to strengthen her hand during the negotiations.
The Conservatives' priorities were set out in a 12-point plan published in January and the letter formally invoking Brexit in March.
The key elements include:
The Labour Party campaigned against Brexit in the referendum but now says the result must be honoured, and is aiming for a "close new relationship with the EU" with workers' rights protected.
The party has set out several demands and tests it says Brexit must meet. These include:
Impact on UK economy
When the election exit poll was revealed, the pound immediately dropped by 2 percent as investors took a position that a hung parliament was a possible outcome, writes BBC's economics editor Kamal Ahmed.
A hung parliament is seen as a sign the government's direction of travel is less certain.
Deals would have to be done. And those vital Brexit negotiations could become all the more difficult.
The future of the UK economy could be confused by a fog of political to-ing and fro-ing, the editor says.