The case for the UK remaining a member of the European Union is "extraordinarily strong", according to Britain's commissioner in Brussels.
Lord Hill warned Eurosceptics "you can't have your cake and eat it", with the trade and business benefits of membership only available if the UK remains part of the union.
David Cameron's plans for an in/out referendum on membership by the end of 2017 received a boost as Labour dropped its opposition to the public vote.
The Prime Minister will host European Commission President Jean-Claude Juncker at his Chequers country retreat tomorrow before carrying out a whirlwind tour of continental capitals to set out his demands for reform.
Financial stability c ommissioner Lord Hill a cknowledged that there was a "long way" to go before a deal could be reached on Mr Cameron's call for renegotiation, comparing exchanges so far to the opening overs of a cricket test match.
He told BBC Radio 4's The World This Weekend: "People who will be making the case in the UK and the EU of the business benefits of our membership, the benefits to Britain for geopolitics and for our diplomacy generally, the benefits to being able to try to deliver meaningful free trade agreements - either with the United States or with China - those to me all seem like an extraordinarily strong case to stay in."
He added: "It's true in politics, as in life, that you can't have your cake and eat it. You can't have easy access to a single market, you can't be part of free trade negotiations, you can't be part of an effective European diplomacy without being a full member of it."
Predicting a lengthy process of diplomacy with European counterparts, he said: " The whole history of negotiation is that one goes through a sequence where people say some things are possible, some things aren't possible and finally at some point you come to a resolution.
"We are a long way away from that. This is like the opening couple of overs of a five-day test match we have had so far. People are interested to know what's going to happen but I think we need to allow some time to elapse, allow the process to continue and then positions will clarify."
Business Secretary Sajid Javid said he remained confident Britain would secure the changes it desired, including limiting access to in-work benefits and tax credits.
He told the BBC's Andrew Marr Show: "The fact we're having this referendum that the British people have voted for - a straightforward in-out referendum before the end of 2017 - I think helps to concentrate the minds now of our European partners, so they will take us seriously on these issues.
"I think we've already seen the start of that process. We're not pretending it's going to be easy, we're going to need some patience but I think we'll get there."
Acting leader Harriet Harman said her party would support the legislation paving the way for the referendum, removing a potential obstacle to the public vote.
It marks an about-turn for the party, which had rejected the idea under Ed Miliband's leadership.
Writing in the Sunday Times, Ms Harman and shadow foreign secretary Hilary Benn admitted voters wanted a say on membership, but warned against a British exit, or Brexit.
They said: " The Labour Party doesn't want to see the UK stumble inadvertently towards EU exit. We will make the case for our continued membership. The notion that Britain's future and prosperity and security lies shutting itself off from this market and a world that is increasingly interdependent makes no sense.
"And in an age of powerful trade blocs, with the growing economies or Asia and Africa, we have more power by being in the EU than we could ever hope to have by acting alone. That is the argument we will make in this referendum, as the British people make their decision."
Labour said it also supported efforts to reform the union, including freedom of movement rules.
"Like many people and businesses, we want reform in Europe - on benefits and the way the EU works - and transitional controls on the free movement of citizens from any new member state wanting to work in Britain," the pair wrote.
"We will hold the Prime Minister to account on these. But the EU itself needs to recognise the growing demand from countries across Europe that want more devolution of power and a recognition that the EU must work for those countries that are and will remain outside the euro."
Warning of the dangers of Brexit, Ms Harman said the UK would just be a "small country" outside the EU.
Stressing the economic and political importance of remaining within the EU, she told the Andrew Marr Show: "If we were outside of Europe we would be a small country, outside of those big, continental building blocks around the globe."
She said that the referendum was a "big constitutional issue on its own" and should not coincide with other elections scheduled to take place, such as those in Wales and Scotland next May.
Ukip leader Nigel Farage said: "The Labour Party has been dragged, unwillingly, to accept the inevitable that there will be an in/out EU referendum in the next two years. But Harman and Benn's position shows quite how little they have learned from their defeat and how much more they have to learn about the European Union debate in the UK.
"They grudgingly accept that it is the will of the British people to have a say on their future, but they make it clear that they will campaign for in, whatever the result of Mr Cameron's negotiations.
"So in reality this isn't a conversion to democracy and the facts of the European argument, merely an acceptance of the inevitable."
The SNP will seek to ensure that 16 and 17-year-olds can vote in the referendum promised by Mr Cameron.
The party also called for a rule that would mean the UK cannot leave the EU unless all voters in all four nations have backed Brexit.
The party's Westminster leader Angus Robertson, writing on the Guardian website, said the voting age should be lowered in the same way it was for the Scottish independence referendum.
He said: " I don't agree with having a referendum on EU membership - but if it is to go ahead, then Cameron has a responsibility to help ensure it can be an enriching and open debate. Young people are our future. It is their UK - and their Europe - so they must have their say.
"Sixteen and 17-year-olds can pay taxes, get married and join the armed forces, so it is only right and fair that they should also be entitled to vote.
"The opposition benches must send a message to this Tory government that they cannot ride roughshod over our future"
He added that the SNP will also try to change the legislation for the referendum to ensure that no constituent part of the UK can be taken out of the EU against its will.
"We will propose a 'double majority' rule, meaning that unless England, Scotland, Wales and Northern Ireland each vote to leave the EU, as well as the UK as a whole, Britain would remain a member state."