Prime Minister David Cameron said the causes of migration must be dealt with, not just the consequences, after the British warship HMS Bulwark launched a mission to rescue at least 500 migrants on Sunday (7 June).

British, Swedish, Spanish and Italian ships are en route to help about 10 migrant boats that called for assistance on Sunday, the Italian coast guard said, following the rescue of thousands of others in the Mediterranean this weekend.

"HMS Bulwark is in the Mediterranean because we want to save lives. Britain is a country that doesn't walk on by – we're a country with a conscience and that's right. But we also need to do more to stop people leaving their countries in the first place," said Cameron.

"That's what we are using our aid budget for, that's what we'll be talking about here at the G7 – how we put a Libyan government together. We need to deal with the causes of this migration, not simply with its consequences."

He also confirmed he would raise the issue of corruption at the G7 summit.

"Corruption is a cancer at the heart of so many of the challenges that we face in our world. We've seen it in football, but also corruption is what stops so many countries from succeeding, corruption is what brings about some of the challenges to our security. So I'm putting it on the table here and saying this is something we must not turn a blind eye to as perhaps was done for too long in the world of football," he said.

Cameron was also asked about reports in Britain's Sunday Telegraph newspaper that more than 50 lawmakers from his Conservative Party are to join a campaign backing Britain's exit from the European Union unless the British prime minister achieves radical changes in the bloc.

The lawmakers will be part of a new group called Conservatives for Britain (CfB), which will support Cameron's bid for reform while urging an end to EU membership unless significant changes are achieved, the paper said.

"Well what we're having is a renegotiation followed by a referendum. It'll be a renegotiation to make sure that Europe works in Britain's national interest and then the referendum which won't be for political parties to decide, it'll be for the British public to decide in an in-out referendum," he said.

Cameron is attempting to persuade European leaders to back UK demands for reform before holding an in-out referendum on Britain's EU membership. He has promised the vote by the end of 2017.