UKIP leader Nigel Farage has pledged to make the Eastleigh by-election a four-way marginal fight and said his party's number one issue will be immigration.
Kicking off the party's campaign in the Hampshire seat vacated by Chris Huhne's resignation for pleading guilty to perverting the course of justice, Mr Farage said: "Our number one issue is that this country faces a second massive migratory wave
"We have nothing against people from Bulgaria and Romania, we wish then well, but we cannot think it's right this country has a total open door policy."
He said the pressure on housing, education and services from immigration "poses a major problem" for the country, especially if people from other countries claim benefits.
Citizens from Romania and Bulgaria will have full movement rights across Europe from 2014.
He also said that the other parties were ignoring the issue of immigration.
"I'm always confident when UKIP is on the ballot paper," Mr Farage said.
"If we can rally our support like we have in the last three by-elections in Rotherham, in Middlesbrough and in Corby then this seat could become a four-way marginal."
He did admit that the Liberal Democrats have the advantage in the constituency but he added that he was "delighted" that the Tory candidate, Maria Hutchings, is Eurosceptic.
Mr Farage also called the EU budget reduction negotiated by Prime Minister David Cameron as "a rotten deal for Britain".