The Paris attacks have made it more important that Britain remains a member of the European Union, David Cameron has said as he made his strongest signal yet that he will hold the referendum in 2016.

The Prime Minister put a noteable emphasis on national security as he told journalists at a press conference in Brussels that he would campaign for an 'In' vote if he succeeds in achieving a "fundamental change" to the UK's relationship with the EU.

He said leaders had made "good progress" on his four demands for change, but admitted there was "a lot of hard work to do" over his most contentious proposal - to make EU migrants wait four years before they can claim in-work benefits.

Mr Cameron predicted 2016 will go down in history as the year Britain's relationship with the EU "fundamentally" changed and set a target of February for finalising a deal with his fellow 27 EU leaders.

If he succeeds in completing negotiations at the February summit, a referendum could be held as early as June, which is believed to be Downing Street's preferred date.

He said continued membership of the EU was important for Britain's economic and national security.

Speaking to journalists at lunchtime on Friday after completing the summit of 28 EU leaders in Brussels, Mr Cameron said: "We've made good progress, we are a step closer to agreement on the significant and far-reaching reforms I have proposed.

"It is going to be tough and there is a lot of hard work to do .

"But I believe 2016 will be the year we achieve something really vital, fundamentally changing the UK's relationship with the EU and finally addressing the concerns of the British people about our membership.

"Then it will be for the British people to decide whether we remain or leave. It is a choice we will all need to think hard about.