The campaign for Britain to leave or stay as part of the European Union (EU) is in full swing ahead of a membership referendum on June 23.

This comes after Prime Minister David Cameron sealed a deal for "special status" in the EU on Friday ( Feb 19) , paving the way for him to push for Britain to stay in the bloc.

But his campaign has got off a tough start as dozens of his Conservative Party members, including London mayor Boris Johnson, back Britain's exit from the EU, also know as "Brexit".

Here's what you should know about the issue:

What's in the Britain-EU deal?

Immigration

What Mr Cameron wanted: To make EU migrants to Britain wait four years before claiming in-work benefits and state-subsidised housing. He also wanted to stop EU migrants from claiming child benefit and sending it back to their families overseas.

What he got: An "emergency brake" for seven years on certain benefit payments to newly arriving EU migrant workers. A system to be implemented will tie the amount of benefits EU migrants in Britain can claim for their children still living in their home country to local conditions there.

Sovereignty

What Mr Cameron wanted: To opt out of the commitment to "ever closer union", a central pillar of the European project, in a "legally binding and irreversible" way. He sought an enhanced role for national parliaments.

What he got: A carve-out on the issue of ever-closer union - treaties will be changed in future to make clear Britain is not committed to this. A "red card" measure would allow a group of national parliaments - making up more than 55 per cent of the 28 parliaments - to be able to veto EU legislation.

Economic governance

What Mr Cameron wanted: To ensure the EU does not use Britain's status as a non-euro zone country against it. He asked for a series of "legally binding principles", including recognition that the EU has more than one currency and that non-euro zone countries should not face discrimination.

What he got: Protections to prevent the City of London from being discriminated against by euro zone states. While Mr Cameron claimed the EU had recognised more than one currency "for the first time", the language in the agreement is vague. European Commission chief Jean-Claude Juncker insisted the deal did not include giving London a veto over euro zone issues. Competitiveness

What Mr Cameron wanted: To cut red tape for business. He urged the bloc to go further in ensuring the free flow of capital, goods and services so as to boost the economy.

What he got: Mr Juncker has made improving EU competitiveness a priority so this area presents less of a problem, with the bloc agreeing to "enhance competitiveness" and take "concrete steps" to improve.

What are some pros and cons of EU membership?

One of the biggest advantages is free trade between member nations, making it easier and cheaper for British companies to export their goods to Europe. Some business leaders think the boost to income outweighs the billions of pounds in membership fees Britain would save if it leaves the EU.

Free movement of people across the EU also opens up job opportunities for British workers willing to travel and makes it relatively easy for companies to employ workers from other EU countries.

One downside of EU membership is Britain is bound by EU rules and regulations and cannot make trade deals with important non-EU partners.

Who supports Britain remaining in EU?

- Mr Cameron will lead the campaign for Britain to stay in the EU.

- Mr Jeremy Corbyn, leader of the main opposition Labour party, has said his party will campaign to stay because of investments, jobs and worker protection from the EU.

- Ms Nicola Sturgeon, Scotland's first minister and head of the pro-independence Scottish National Party, has said she will be at the "forefront" of efforts to stay in, amid warnings that a " Brexit" could trigger a second vote for Scotland to leave the United Kingdom.

- Key business leaders including Sir Richard Branson have spoken out for EU membership, with the Virgin founder warning that a departure would be a "very sad day". Barclays bank chairman John McFarlane has warned that London, Europe's premier financial hub, would be "significantly" worse off without the EU.

- Several cultural figures are also pro-EU. Award-winning British actress and campaigner Emma Thompson has said leaving Europe would be "a crazy idea". "Of course I'm going to vote to stay in Europe. Oh my God, it would be madness not to," she told reporters at the Berlin Film Festival.

- Prince William has never directly addressed the issue but in a speech at the foreign ministry in London in February, the second in line to the throne made some comments that were widely interpreted in British media as being in favour. "Our ability to unite in common action with other nations is essential, it is the bedrock of our security and prosperity," the prince said.

What are some pros and cons of Brexit?

Britain can secure trade deals with other countries such as China, India and America. The 350 million pounds (S$708 million) saved in EU membership can be spent on scientific research and new industries. The country can also take control in areas such as employment law, health and safety.

The down side is Britain will still be subject to the politics and economics of Europe, but would no longer have a seat at the table to try to influence matters. The country now accounts for less than 1 per cent of the world's population and less than 3 per cent of global income (GDP). Each year that goes by, these numbers shrink a little. The country will find it increasingly hard to get its voice heard on topics that affect its prosperity and well-being if it goes it alone.

A study by the think-tank Open Europe, which wants to see the EU radically reformed, found that the worst case Brexit scenario is that the British economy loses 2.2 per cent of its total GDP by 2030. However, it says that GDP could rise by 1.6 per cent if Britain could negotiate a free trade deal with Europe and pursued "very ambitious deregulation".

Who wants Britain to leave the EU?

- Five of the cabinet's 22 members have so far come out in favour of Brexit:Justice Minister Michael Gove, Work and Pensions Minister Iain Duncan Smith, Northern Ireland Minister Theresa Villiers, Sports and Media Minister John Whittingdale and Chris Grayling, the cabinet's representative in parliament.

- Mr Nigel Farage, head of the anti-EU UK Independence Party (UKIP), has advocated for Britain to leave the EU all of his political life. He tweeted: "June 23rd: our golden opportunity. Let battle be joined. We want our country back". He said life outside of Europe would mean "control of our borders, global trade deals, making our own laws. An exciting future".

- Some entrepreneurs have spoken out for Brexit, including vacuum cleaner tycoon James Dyson. The inventor was quoted by the Daily Express as saying that he did not want to stay in a Europe "dominated and bullied" by Germany.

- Only very few cultural figures have declared public support for Brexit. Oscar-winning actor Michael Caine has said Britain should leave if it does not manage to negotiate "extremely significant changes". "I sort of feel certain we should come out," the 82-year-old told BBC radio.

SOURCE: AGENCE FRANCE-PRESSE, THE GUARDIAN, THE WEEK, THE ECONOMIST