Britain is facing its biggest political decision in decades: Remain a member of the European Union, or walk away from the world's largest single market?
U.K. citizens worldwide will vote in the historic referendum on June 23. Prime Minister David Cameron will campaign for the U.K. to stay in the EU.
The British economy is the second largest in the EU. Its decision on whether to stay or go will have big implications not only for the people of the U.K. but also global financial markets and the future of Europe.
Related: Bank of England chief warns of Brexit risks
1. Campaign will last 10 weeks
The official campaign began on Friday and will run for just under 10 weeks, ending on June 22.
Electoral officials have designated a lead campaign group for each side of the debate: Britain Stronger in Europe, and Vote Leave.
Political parties, other groups and even businesses can join the argument, but under certain restrictions.
2. How much can the campaigns spend?
There's no universal limit on how much can be spent making the case for, or against, Brexit.
But spending by the lead campaigns will be capped at £7 million ($10 million). They have to publish details of any individual loan or donation over £7,500 -- the first such declaration is expected around May 11. They'll also receive a grant to help with costs such as TV broadcasts and a mailing to voters.
Political parties can spend cash by contributing to the lead campaigns, or registering as campaigners. Either way, they can only spend up to limits based on their share of the vote in the last general election. That means Cameron's Conservative Party could spend up to £7 million, and the main opposition Labor Party £5.5 million. Labor leader Jeremy Corbyn wants Britain to remain an EU member.
Individuals, businesses and other organizations are free to campaign if they spend less than £10,000. If they want to spend more, they have to register with the Electoral Commission, and their spending will be capped at £700,000.
Related: The truth about UK immigration
3. Companies have to tread carefully
Businesses, banks and research organizations have been sharing their views on the pros and cons of EU membership. Now the formal campaign is underway, they'll have to be careful not to engage in activities that could mean they breach the spending rules.
To help them navigate this minefield, electoral officials have issued guidelines. And they're already having an impact. Take this research report from Deutsche Bank (DB) warning of the risk of a Brexit:
"This report was published and made available before 15 April. The report is for Deutsche Bank clients, not the broader voting public in the UK. Deutsche Bank does not intend to promote a particular outcome to the UK referendum. Readers should vote as they personally see fit," reads the disclaimer at the top.
4. Who can vote, and what's the question?
British and Commonwealth citizens over 18 who live in the U.K., and U.K. nationals who have lived overseas for less than 15 years, will be able to vote in the referendum. As will citizens of Gibraltar.
EU citizens living in the U.K. -- except those from Ireland, Malta and Cyprus, which all have historical ties with Britain -- will not have a vote.
Eligible voters will be asked: "Should the United Kingdom remain a member of the European Union or leave the European Union?"
5. When will we know the result?
Counting will begin at 10 p.m. local (5 p.m. ET) on June 23 and continue overnight.
The result will be declared as early as possible on June 24. The announcement will be made in Manchester, at the regional counting center for the northwest of England.
6. What happens then?
If Britain votes to leave the EU, Prime Minister Cameron will have to inform other European leaders formally, triggering two years of complex exit negotiations.
Cameron says he will oversee those talks, but he is almost certain to face calls from within his own party to quit.
If Britain votes to remain, Cameron will have to make a reformed relationship with the EU work.
