British prime minister David Cameron has dismissed the defection of MP Mark Reckless to Ukip as "senseless and counter-productive" as Tory divisions over Europe overshadowed the start of the Conservative Party Conference.
The prime minister has been rocked by a double blow, with Mr Reckless's switch to Nigel Farage's "people's army" and the resignation of government minister Brooks Newmark over a sex scandal.
Mr Cameron insisted only a Tory government could deliver a referendum on membership of the European Union and said the defection of Mr Reckless was "frustrating".
On BBC1's Andrew Marr Show he insisted he would renegotiate the UK's relationship with Brussels before the referendum promised by the end of 2017, with a focus on changing the immigration rules.
“If I don’t achieve that it will be for the British public to decide whether to stay in or get out,” he said.
But he added: “I have said this all my political life: if I thought that it wasn’t in Britain’s interest to be in the European Union, I wouldn’t argue for us to be in it.”
The Tory gathering in Birmingham has been rocked by the scandal involving Mr Newmark, who resigned after reportedly sending explicit pictures of himself online to an undercover newspaper reporter in a tabloid sting, and the loss of a second MP to Ukip.
Rochester and Strood MP Mr Reckless followed Clacton MP Douglas Carswell to Ukip, in a sign of the unease within Tory ranks.
Mr Cameron said he had “not specifically” been aware of Mr Reckless’ intentions but “he very rarely votes for the government and has made his views known”.
The prime minister acknowledged that Mr Reckless’ actions made a Conservative government “less likely” after 2015.
He added: “These things are frustrating and, frankly, they are counter-productive and rather senseless.
“If you want to have a European referendum, if you want to have immigration controlled, if you want to get the deficit down, if you want to build a stronger Britain that we can be proud of, there is only one option and that is to have a Conservative government after the next election.”
The choice was between the Tories and Labour to form the next government, he said, "so to act in a way that makes the Conservative government less likely is senseless and counter-productive".
Mr Cameron rejected the suggestion he was “not a proper Conservative” and added: “Douglas Carswell and Mark Reckless are both people who, unlike me, they want to leave the European Union no matter what.
“I don’t agree with that. I think we should have a real go at reform to get a better deal for Britain.
But then there is this promise - and it is a complete and clear promise - of an in/out referendum by the end of 2017.
“So, even if you don’t agree with my renegotiation strategy, I’m the only Prime Minister who is going to give you the chance to have a vote to stay in or get out of the European Union.”
Mr Cameron said immigration would be “at the heart of my renegotiation strategy”, including tightening the rules on freedom of movement within the EU. “Of course there is a right to go and work in other European countries, but it is not an unqualified right,” he said.
“You should not be able to move for benefits, you should not be able to abuse the rules, when new countries join the European Union you should be able to say to them ‘look, your people cannot come and work in our country until your economy is at a much more similar level to ours’.”
The prime minister added: "The right to go and work in France or Spain, or Spanish people to come and do a job that has been advertised here, that is one thing. "But it's a completely different thing to be able to go, to claim benefits, actually even to work in Britain but to send your benefits home to your families that stay in your home countries. All of those things need to be addressed."
Mr Cameron has faced pressure from within the Tory ranks anxious about the threat posed by Ukip, to state that he would campaign for the UK to leave the EU if he was not able to secure a better deal.
The prime minister said: “I am determined we will do the right thing for Britain ... What I believe is right for Britain is to seek reform, get reform and get Britain to vote to stay in a reformed European Union.
That is my plan.” He added: “When you are going into a negotiation you should set out what you want to achieve. Also, I want the 27 other countries in Europe to see that there is a plan here that, with reform, can end with a reformed European Union and a reformed relationship with Britain and Britain staying in. I want them to see that is the goal.
“If I don’t achieve that, it will be for the British public to decide whether to stay in or to get out.” Pressed on whether there were any circumstances in which he would recommend severing ties with Brussels, he said: “If I thought it wasn’t in Britain’s political interest to be in the European Union I would not argue for us to be in it.
“I’m a deeply patriotic politician and person. I do this job because I love my country, I care passionately about its future and I want it to be a strong, proud, self-governing, independent nation.”
Asked again whether, if he could not get the desired outcome from his renegotiations he would recommend an exit from the EU, he said: “I believe we will get what we want. I think the British public want someone who is absolutely determined to deliver, for them, a better future in Europe.
“But in the end, I don’t decide; they decide.” Following the defections by Mr Carswell and Mr Reckless, there has been speculation that other Tories could be tempted to join Mr Farage’s party.