The Prime Minister has met French President Francois Hollande in Paris to discuss potential changes to the EU treaty which he says could benefit both countries.
Describing France as an "essential and valued partner of Britain", David Cameron said the two countries might have different priorities, but similar aims for the future, and said he hoped they could reach an agreement - including on issues such as tackling extremism and climate change.
France is an essential and valued partner of Britain. Our economies are interwoven, and we are two major military powers in the EU, with global reach and shared values.
My priority is to reform the European Union to make it more competitive, and address the concerns of the British people about our membership. The status quo is not good enough, and I believe there are changes we can make which will not just benefit Britian, but the rest of Europe too.
Of course, the priority for Francois is to strengthen the Eurozone, to ensure a successful single currency, and Britain supports that.
We have different priorities, but we share a common objective - to find solutions to these problems.
Foreign Secretary Philip Hammond has warned that the British people are likely vote to leave the EU unless there is substantial reform in Brussels.
He spoke as David Cameron began a whistlestop tour of Europe, meeting political leaders in a bid to build support for change.
ITV News deputy political editor Chris Ship reports:
David Cameron said the UK and the Netherlands were "old friends and like-minded allies" as he met with Dutch prime minister Mark Rutte in The Hague.
Among the issues to be discussed will be European reform, the need for flexibility and the need to focus on growth and jobs, Mr Cameron said.
The Hague is the first of four European capitals the Prime Minister is visiting on a whirlwind tour to discuss his plans to renegotiate the terms of the UK's membership of the European Union with key leaders.
Later Mr Cameron will travel to Paris in France where he will meet with President Hollande.
The Government will not win its referendum on Britain's continued membership of the European Union unless there is "substantial" reform in Brussels, Foreign Secretary Philip Hammond has warned.
Speaking to the BBC, Hammond said: "The Prime Minister is very clear in dealing with European counterparts that if we are not able to deliver on these big areas of concern that the British people have, we will not win the referendum when it comes.
"We expect our European Union partners to engage with us in delivering a package that will enable the British people to decide that Britain's future is best delivered inside the European Union.
"We expect that some of our partners will adopt a hard line at the start of the negotiations - that's how negotiation works - but we are very confident that, over the course of the summer and perhaps onwards through the winter, we will be able to negotiate a substantial package of reform which will address the concerns that the British people have."
Prime Minister plans a whistle-stop charm offensive with EU leaders this week.Read the full story ›
The SNP will make "a positive case" for keeping the UK in the European Union, one of its new MPs has told Parliament.
In his maiden speech to the Commons, Stephen Gethins, MP for North East Fife, also said that his party would try and get as many people voting as possible.
He said: "We want to look at a positive case, even look at some areas where we could be deepening our relationship with our European partners."
He added that "the Scottish referendum provided many lessons", especially "including as many of our citizens as we possibly can in a debate about the future of our respective nations".
Ukip leader Nigel Farage has criticised the wording of the question that will be asked at the EU referendum.
Because David Cameron "is opting to give the pro-EU side the positive 'Yes'", he said, "suggests strongly that his negotiations are so much fudge".
"He has already decided which way he wants the answer to be given, without a single power repatriated."
Details of the In/Out referendum will be presented to Parliament in a Bill this morning.Read the full story ›
A Labour eurosceptic should lead the "out" campaign at the EU referendum, the party's biggest donor has said.
John Mills, the founder of JML, said that Ukip leader Nigel Farage would put many potential backers off, and suggested ex-minister Kate Hoey as a suitable candidate, as well as Labour backbenchers Graham Stringer and Kelvin Hopkins.
Mr Mills, a vocal critic of Brussels, said that "a successful no campaign has to be a cross-party campaign".
"If it is all the Conservative Party and no Labour then it is never going to gel so it is really important to have key Labour figures in it," he said.
He admitted he would still vote to stay if David Cameron could secure sufficient reforms.