David Cameron has denied he is a "closet Brexiteer" as he insisted the UK was better off in the European Union.

The British prime minister was speaking as he got a boost in his campaign to keep Britain in the 28-member bloc after leaders at the G7 summit warned Brexit would be a "serious risk" to world growth.

The joint declaration from the leaders of the US, UK, Japan, Germany, France, Italy and Canada included a passage saying "UK exit from the EU would reverse the trend toward greater global trade, investment and the jobs they create and is a further serious risk to growth".

The statement at the end of the two-day Ise-Shima summit follows similar heavyweight interventions from the International Monetary Fund, Organisation for Economic Co-operation and Development, G7 finance ministers and previous comments from Barack Obama.

It came as "purdah" rules came into force on Friday, preventing British government departments from issuing 'pro-Remain' guidance.

Speaking at a press conference in Japan, Mr Cameron also gave more details about plans to send a British warship to help tackle people and arms smuggling off the coast of Libya.

But he refused to comment on reports the SAS had blown up an Islamic State suicide truck in the Libyan city of Misrata earlier this month.

Pressed over claims by his former senior adviser Steve Hilton that he is a "closet Brexiteer", Mr Cameron said:

"It's for me all about Britain's national interest. To me this is not an argument about Europe, yes or no.

"It's an argument about Britain big and bold. I have consistently throughout my political life taken the view that for all its imperfections, let's get stuck in there, change the things we don't like.

"But I actually think, and I said this throughout the renegotiation, the best outcome for us was for Britain to stay in a reformed European Union.

"So no, I have never been a closet Brexiteer. I am absolutely passionate about getting the right result, getting this reform in Europe and remaining part of it.

"It's in Britain's national interest, that's what it's all about."

The mention of Brexit in the official G7 declaration represents a victory for Mr Cameron, but German Chancellor Angela Merkel said the referendum was not discussed during formal conversations at the two-day summit.

She said: "It was no subject here. But there was the signal that all who sat here want Britain to stay part of the EU."