Mr Spink says he will continue in elected politics 'for a long time'
The UK Independence Party has its first MP after former Conservative Bob Spink announced he is to join its ranks.
Mr Spink resigned the Tory whip last month in a row over apparent efforts by his local party to deselect him.
He claims the Conservatives have been "dishonest" over their Europe policies and says a by-election in his Castle Point constituency is unnecessary.
UKIP say they are "delighted" about their new recruit. Senior Tories say they are "relaxed" about the move.
Mr Spink said he had decided to leave the Conservatives after years of being "disenchanted" with the party.
He accused them of being "dishonest" with the public over a wide range of issues, including Europe, pensions, crime, immigration and defence.
"I voted against the Tory whip 22 times in the last session, on issues such as equality for women on pensions, which I think is a good thing, and on defence issues, and getting tough on crime, issues that really concern the people of this country," he told BBC's Five Live Breakfast.
"I think that politics is becoming increasingly dishonourable at Westminster and I am delighted to be joining a strong UKIP team so that we can get some meat on these domestic policies."
UKIP leader Nigel Farage said: "I am delighted to welcome the hard-working and deeply principled Bob Spink as UKIP's first Member of Parliament.
"As Bob is a signatory to the 'Better Off Out' campaign, joining UKIP was a logical step."
UKIP peer Lord Willoughby de Broke said: "I am thrilled that UKIP now has a representative in the House of Commons.
"I look forward to working with him to further UKIP's programme of leaving the European Union."
Mr Spink has sometimes been a controversial figure who fell out with the Conservative leadership following bitter rows with his local constituency.
In February he claimed members of his local Conservative Association had been working to deselect him.
During the 2005 general election campaign he was accused of whipping up racial tension for running a newspaper advert on immigration, headlined: "What bit of 'send them back' don't you understand Mr Blair?"
Mr Spink is the third Conservative MP to lose the Tory whip in the last year. The others were Derek Conway and Andrew Pelling. Quentin Davies defected to Labour in June 2007.
Mr Spink was first elected to Parliament in 1992, but lost his seat in 1997, before regaining it in 2001.