AP EXPLAINS: What's behind Britain's wrangling with the EU?

The grouping became the EU in 1993 and has grown into a 28-nation bloc of more than 500 million people stretching from Ireland to the Aegean Sea, with substantial powers over member states' laws, economies and social policies.

Britain joined the bloc in 1973, but many Britons feel their island nation — a former imperial power with strong ties to the United States — is fundamentally different to its European neighbors.

Labour Party leaders also argue that European rules and regulations and European courts have improved the way British workers are treated.

The "remain" side, headed by the prime minister, argues that Britain's economy would suffer a tremendous blow if the country leaves the trading bloc.

The "leave" side has focused on concerns about immigration from other parts of the EU since membership in the bloc gives residents of other countries the right to live and work in Britain.

[...] they have offered Britain an "emergency brake" that will allow it to restrict social welfare benefit payments for a period if it can show its welfare system is under pressure.