Ukip has its first elected MP after Douglas Carswell took almost 60% of the vote in the Clacton by-election.
The former Tory MP, who triggered the vote after defecting from the Tories, said there was "nothing we cannot achieve" if Ukip was able to extend its appeal to "all Britain and all Britons".
He won the seat with a majority of 12,404 over the Tory candidate Giles Watling and his victory capped a dramatic night for Ukip which saw Nigel Farage's party run Labour close in the Heywood and Middleton contest.
Party leader Nigel Farage said:
Mr Carswell won Clacton for the Tories in 2010 with a majority of 12,068 and a vote share of 53% - he increased both after switching to Ukip.
His 21,113 votes saw him comfortably defeat Mr Watling, on 8,709 votes, with Labour in third place and the Greens pushing the Liberal Democrats down to fifth - losing their deposit.
In his acceptance speech Mr Carswell explained his decision to stand down after defecting from the Tories.
UKIP leader Nigel Farage indicated there could be further defections to UKIP in the months running up to the general election - without necessarily forcing a by-election.
"There comes a point where there will be backbench Conservatives, and perhaps some Labour ones too, who will reckon they have got a better chance on a UKIP ticket next year than a Tory or Labour ticket," he told Sky News.
"I don't think we are quite at that moment yet."
Asked if they would be expected to resign as an MP, as Mr Carswell and Mr Reckless had done, Mr Farage said that might not be the case closer to the general election.
"There is generally a six-month convention with these things, so if people were to come to us later than that there might not be."
Tory minister Priti Patel, who is the MP for Witham in Essex, said the result was disappointing but said people "don't necessarily vote along traditional party lines" at by-elections and Mr Carswell had benefited from being the incumbent.
Turnout was 51.2% with 35,386 ballot papers counted, a decrease from the 64.2% turnout at the 2010 general election.
Tory Giles Watling received 8,709 votes to finish second while Labour's Tim Young finished third with 3,957 votes.
UKIP defector Mark Reckless, who has also triggered a by-election in Rochester and Strood, attended the count in Clacton's Princes Theatre.
Former Liberal Democrat MP Lembit Opik also made an appearance in support of independent candidate Charlotte Rose, who was campaigning for sexual freedom and finished in last place with 56 votes.
Mr Carswell previously secured the Clacton seat in 2010 with a majority of 12,068 votes over Labour's Ivan Henderson, having first being elected to Parliament in 2005 in the Harwich seat.