Difficult negotiations setting out the new relationship between Britain and its European neighbors will continue throughout 2020.

This series of stories chronicles Britain's tortured relationship with Europe from the post-World War II years to the present.

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With Tony Blair's departure as prime minister in 2007, pro-Europeans lost a key voice at the heart of government. His successor, long-time Treasury chief Gordon Brown, was far more lukewarm. He even turned up late for the signing of the Lisbon Treaty, which amended the existing EU Treaties and included for the first time a mechanism by which a country could leave the EU.

For the record, Article 50 of the Lisbon Treaty stated that any member state "may decide to withdraw from the Union in accordance with its own constitutional requirements." Its author Lord Kerr didn't think the U.K. would be the country to use it.

Brown had other things on its mind. His time at the helm was marked by the greatest peacetime shock to afflict the global economy since the depression of the 1930s. Following the collapse of U.S. investment bank Lehman Brothers in September 2008, the financial crisis went global.

Banks around the world, including big U.K. names such as Lloyds and Royal Bank of Scotland, had to be bailed out by the taxpayer. Britain endured its deepest recession since World War II. Though Brown won plaudits for his handling of the crisis, he lost the 2010 general election, ushering in a coalition between a euroskeptical Conservative Party and the very pro-EU Liberal Democrats.

With many in his party increasingly vexed by EU membership and with the UK Independence Party making headway with its demand for a fresh vote on Britain's membership of the EU, Prime Minister David Cameron was under pressure to show his euroskeptical teeth. In 2011, he vetoed an EU treaty designed to help stabilize the euro, which was fighting for its life following a debt crisis, particularly in Greece. Cameron eventually decided that a referendum promise would help bind his party — much like Harold Wilson had done with his Labour Party back in 1975.

Following the Wilson playbook, Cameron said in a speech in January 2013 that another referendum should take place after a renegotiation that would bring back powers to Britain from the EU. He said democratic consent for the EU in Britain was "wafer thin" and that another referendum was needed.

"I believe in confronting this issue, shaping it, leading the debate," he said. "Not simply hoping a difficult situation will go away."

The pledge though depended on the Conservatives winning a majority at the ensuing election, something they hadn't done since 1992. Most pollsters thought the election would result in another hung Parliament or of Labour returning to power. In the event, Cameron did win that Conservative majority.

Another referendum was on.