The prime minister has presented MPs with his case for Britain to stay in the EU.
David Cameron's campaign comes hours after Boris Johnson he would be supporting the Out campaign.
MPs from across the South and South East have been declaring whether they want to stay or leave the EU.
Royston Smith, the Conservative MP for Southampton Itchen, has explained why he will campaign for a vote to leave the EU.
“I have spent my entire political life campaigning for a referendum on our relationship with the EU which is why I was thrilled when David Cameron committed to give the public a say," he said in a statement on his website.
"I supported The Prime Minister’s efforts to secure a new, more flexible arrangement with the EU but it is now obvious we will never manage to agree a deal which would answer the fundamental questions of sovereignty and repatriation of powers to the UK Parliament.
"There are those who want to remain in the EU regardless of a successful outcome to the Prime Minister’s renegotiation and they are entitled to hold that position.
"But I believe they are misguided and the time has come to have the confidence to chart our own course.
"I hope we can have a good mannered debate on our future with the EU and a conclusive outcome which both sides will accept.
"It is in everyone’s interest to secure the very best outcome for the British people and I believe that will be best achieved in withdrawing from the overbearing EU structures while remaining good neighbours to our European friends.”
Boris Johnson and David Cameron are now clear rivals in the battle for Britain's future in Europe. But who else is on their side?
When David Cameron was finally persuaded to back an in/out referendum, it was designed to tackle UKIP head on.