Six in ten people who voted Ukip in the European elections did so as a protest, according to polling from the influential Conservative, Lord Ashcroft.
Writing on the Conservative Home website, the peer said his survey of 4,000 voters found that 60% of Ukip voters had said they were voicing anger at the party they usually vote for or registering a more general protest.
However 80% of Ukip voters also said they thought Nigel Farage's party had the best policies on Europe.
The results of Thursday's European Parliament vote will be revealed tomorrow.
UKIP's successes in the local elections have raised the possibility of them winning a Westminster seat at the general election.
A year ahead of the general election, Ed Miliband's party is seeing only patchy support in areas it should expect to gain ground.
UKIP are likely to make Rotherham a key target for 2015 after a storming result in the local elections.