Election Results: Nigel Farage warns over new referendum
Nigel Farage has said he would have "no choice" but to return to politics if Brexit was under threat, as UKIP look unlikely to win any seats.
The former UKIP leader said "we may be looking down the barrel of a second (EU) referendum".
The BBC general election forecast suggests that the Conservatives will be the largest party, but finish just short of having a Commons majority.
Party leader Paul Nuttall said Theresa May had put Brexit "in jeopardy".
He said he believed this election was wrong from the start, adding: "Hubris."
Deputy Chairwoman Suzanne Evans described the poll as "shocking" and said if right, "Brexit at risk and Marxists at large".
UKIP had no MPs at the end of the last parliament, after Douglas Carswell left the party in March.
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Mr Farage - who resigned as party leader in July of last year - said Jeremy Corbyn had managed to get UKIP voters around the UK to vote for him.
He said: "I think the shock we're seeing tonight is all about personality. UKIP voters want someone who speaks for them. Corbyn looked comfortable in his own skin."
He added that Mrs May's credibility as party leader had been "fatally damaged".
'Hell to pay'
Neil Hamilton, UKIP's leader in the Welsh Assembly, said his party's vote had been "squeezed" as the election had become a "binary competition between Labour and the Tories".
He said that after this "disastrous election" for the Conservatives, "we'll be able to carve out a niche" for UKIP in UK politics.
After seven election results had been announced, it was confirmed the party had lost their deposit in five seats, meaning they failed to achieve 5% of the vote share.
UKIP economy spokesman Patrick O'Flynn said he believes there will be "hell to pay" with Eurosceptic voters if the exit poll turns out to be correct, adding he expects people to "gravitate back to UKIP in very large numbers".
Mr Nuttall's party stood in around half the number of seats contested in 2015.
The UKIP leader stood in Boston and Skegness, but failed to take it from the Conservatives.
Conservative sources say the party is to win Clacton, the seat won at the 2015 election by Mr Carswell for UKIP.
A total of 650 Westminster MPs will be elected, with about 45.8 million people entitled to vote.