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.
A leading independent employers' organisation has called for the Prime Minister to be "ambitious with business".
The Confederation of British Industry urged the new Government to set out clear plans for the country's economic growth and to take action within the first 100 days.
It said that business will take an active role in "arguing the case for the UK to remain inside a reformed EU" and argued that it is "vital" for the Government to set the bar for that EU reform at an "ambitious and achievable" level.
The new Government must get into its stride quickly. It should set out clear plans for the next parliament within the first 100 days, and have a laser-like focus on delivery.
The Prime Minister should prioritise building on the progress made to get the deficit down, finding more innovative ways to deliver public services and backing the final decision from the Airports Commission so we get diggers in the ground by 2020.