LONDON (Reuters) - The United Kingdom voted in 2016 to leave the European Union but it is still uncertain when or if that will happen.

FILE PHOTO: Former Prime Ministers Lady Thatcher and Edward Heath listen to speeches at the 115th Conservative Party conference in Bournemouth in this file photograph dated October 7, 1998. REUTERS/Dylan Martinez/File Photo

Below is a timeline of its tortuous journey in and out of the European project:

WHY DID BRITAIN SHUN THE ORIGINAL BLOC?

London declined to join the European Union’s forerunner, the European Coal and Steel Community (ECSC), when it was founded in 1952.

Labour Prime Minister Clement Attlee told parliament in 1950 his party was “not prepared to accept the principle that the most vital economic forces of this country should be handed over to an authority that is utterly undemocratic and is responsible to nobody”.

There was also concern it might prejudice close ties with the United States and the Commonwealth group of mainly former territories. Britain also stayed out of the European Economic Community (EEC) when it was formed from the ECSC in 1957.

Conservative Prime Minister Harold MacMillan reversed this position in 1961 and sought membership of the EEC.

With Europe divided by the Cold War, he said the promotion of European unity and stability through the bloc was “so essential a factor in the struggle for freedom and progress throughout the world”.

But France led resistance to Britain’s membership in the 1960s, with Charles de Gaulle blocking Britain’s accession in 1961 and 1967, accusing the British of “deep-seated hostility” to the European project.

WHEN DID BRITAIN FINALLY JOIN?

Britain joined the EEC in 1973 after France dropped its objection’s following de Gaulle’s resignation in 1969.

As he signed the treaty taking Britain into the common market, Conservative Prime Minister Ted Heath said “imagination will be required” to develop its institutions while respecting the individuality of states.

1975 - BRITAIN’S FIRST EUROPEAN REFERENDUM

In 1975, new Labour Prime Minister Harold Wilson, faced with splits among his ministers on Europe, decided to hold an “in-out” referendum on membership. He backed staying in after saying a renegotiation on terms of membership had “substantially though not completely” achieved his objectives.

Britons voted 67 percent to 33 percent to stay in the European Union in 1975.

WHY DID THE 1975 VOTE NOT SETTLE THE ISSUE?

Although new Conservative leader Margaret Thatcher backed the campaign to stay in the bloc in 1975, her party become increasingly divided by the issue and her own relationship with European leaders was tense at times.

She attacked the idea of a single currency and too much power being centralized in EU institutions, telling the then-Commission President Jacques Delors “no, no, no” over his plans for more European integration in 1990.

However, days later she was challenged for the leadership of the party by pro-European Michael Heseltine, and was forced from office when she failed to beat him outright in November 1990.

Her successor, John Major, was forced to pull sterling out of the European Exchange Rate Mechanism (ERM) on so-called “Black Wednesday” - Sept. 16, 1992. The ERM had been intended to reduce exchange rate fluctuations ahead of monetary union.

Major was also beset by divisions over Europe, describing three euroskeptic cabinet ministers as “bastards” in 1993 after narrowly surviving a confidence vote over the EU Maastricht Treaty.

After Labour’s Tony Blair won the 1997 election, his finance minister, Gordon Brown, effectively ruled out euro entry by setting out five economic tests that had been worked out with his top aide, Ed Balls, in a New York taxi.

CAMERON’S GAMBLE BACKFIRES

The tenure of the next Conservative Prime Minister, David Cameron, was also, ultimately, defined by Europe.

The Conservatives returned to office in 2010 after 13 years of Labour government.

In a bid to shore up support in the face of a split party and the small but staunchly euroskeptic UK Independence Party (UKIP), Cameron promised an “in-out” referendum on a renegotiated deal on membership in the party’s 2015 election manifesto.

Cameron said he was satisfied that negotiations with the EU gave Britain enough for him to back a “remain” vote.

But though Britain’s biggest parties backed the campaign to stay in, the people voted to leave by 52 to 48 percent on June 23, 2016. Cameron resigned the morning after the vote and was replaced by Theresa May.

MAY DAYS

May triggered Article 50, the formal EU divorce notice, in March 2017, setting the exit date of March 29, 2019 for Britain to leave - with or without a deal.

In a bid to gain backing for her Brexit plan, she called a snap election for June 2017. The gamble backfired. She lost her parliamentary majority and formed a minority government, supported by the euroskeptic Northern Irish Democratic Unionist Party (DUP).

On Nov. 13, she reached agreement on the terms of Britain’s departure from the bloc with EU leaders.

Lawmakers voted 432-202 to reject the deal on Jan. 15 in the biggest parliamentary defeat for a government in modern British history.

THE FINAL COUNTDOWN?

May dashed to Strasbourg on March 11 and in a late night news conference announced she had secured legally binding reassurances on the so-called Northern Irish border backstop, an insurance policy aimed at avoiding post-Brexit controls on the United Kingdom’s border with EU-member Ireland.

Many Brexiteers and the DUP fear the backstop will trap the United Kingdom in the EU’s orbit, and have sought guarantees it will not.

But the new documents were not enough to sway the most euroskeptic wing of her party and the DUP, and the deal was voted down again on March 12.

In the following days lawmakers voted to avoid a “no-deal” Brexit and to ask for a delay to Brexit.

By law, Britain is due to leave at 2300 GMT on March 29, 2019, the end of next week. But May has said that she will ask for a delay until June 30 if parliament approves a deal by Wednesday.

“Such a short technical extension is only likely to be on offer if we have a deal in place,” May said on March 13.

“The House has to understand and accept that, if it is not willing to support a deal in the coming days, and as it is not willing to support leaving without a deal on 29 March, then it is suggesting that there will need to be a much longer extension to Article 50.”

(Corrects date in first paragraph of final section.)