Nigel Farage has hailed his party's success in the local elections, saying it had put the "Ukip fox in the Westminster henhouse".
UKIP gained more than 150 English council seats, as the Conservatives and Liberal Democrats suffered big losses.
Labour took the most seats, but Ukip also ate into their support to add pressure on leader Ed Miliband.
ITV News Political Editor Tom Bradby reports.
ITV News Political Editor Tom Bradby says Ukip can cause trouble for the established parties in the general election, while he predicted that Ed Miliband could switch to a more radical approach in the months ahead.
An investigation has been launched after a councillor allegedly complained that a voter with learning difficulties "shouldn't be voting".
Officials at Trafford Council in Manchester say they have been made aware of the incident, which allegedly involved an unnamed councillor at a polling station yesterday.
The accusation was made by Kate Green, Labour MP for Stretford and Urmston and shadow spokesman for disabled people.
Disabilities charity Mencap said the allegations were concerning.
Ukip has so far gained 155 seats in the local elections, but in terms of councils Ukip still don't have control of any.
Labour has gained 275 seats, but not as many as they had hoped.
With parties sharing votes in many places, the number of councils where no one party has overall control has risen to 28.
Ed Miliband has defended Labour's election strategy amid public criticism from his own MPs, with some backbenchers claiming the party leadership ignored the threat from Ukip.
Labour has made modest gains in the English council elections but has failed to secure enough support to put it on course for an outright general election victory.
It went into the polls on the back of a torrid week of publicity in which Miliband under-estimated the price of his family's weekly food shop, got caught out identifying a Labour councillor in a regional radio interview and was photographed eating a bacon sandwich in an unflattering manner.
"I think we ran a good campaign but I think to understand what happened last night you have got to go back not for a few weeks but for years to the way people feel this country works," Mr Miliband insisted.
He added: "I believe that we can persuade those people that Labour can offer answers to them and to the challenges they face in their own lives and that's what I'm determined to do over the coming year."
Shadow Chancellor Ed Balls said the results were "not good enough yet for Labour", while backbencher John Mann said the leadership's electoral strategy had been "disastrous".
Labour has gained overall control of Harrow Council after winning nine seats.
The poll saw Ukip lose its only seat in the borough, along with nine other independent candidates, while the Conservatives gained one.
Harrow Council, formerly governed with no party having overall control, now has 34 Labour, 26 Conservative, 2 Independent and 1 Liberal Democrat.
UKIP's successes in the local elections have raised the possibility of them winning a Westminster seat at the general election.Read the full story ›
ITV News Election Analyst Professor Colin Rallings has said Ukip's breakthrough is the "most significant trend in politics for 30 years."
He said: "I remember back in the early 1980s it was the Social Democratic Party, a break away group from Labour, did well in the elections and in the polls.
"They soon joined up with the Liberal Democrats so we became three parties again.
"But now we have a four party situation and in that respect Mr Farage is absolutely right. It makes thing very uncertain as to how the pattern of party loyalties will play out."
Read more: Labour in biggest danger of disappointment
UKIP deputy leader, Paul Nuttall, says his party is the only opposition to Labour in the North of England:
The Labour Party has won a majority in Crawley, with new council leader Peter Lamb becoming one of the youngest in the country at the age of 27.
The party also gained overall control of Amber Valley in a nail-biting count which went down to the very last seat.
Great news from Amber Valley. Now Labour-run after for the first time in 14 years. Congratulations to the whole team there
The wins mean Labour has gained overall control of seven councils so far.