LONDON (AP) — Britain's European Union referendum campaign has officially begun — though Britons could be forgiven for thinking it has been going on for months.

Friday marked the start of the official 10-week campaign ahead of a June 23 vote on whether Britain should leave the 28-nation bloc. The opposing sides — Britain Stronger in Europe and Vote Leave — unveiled billboards, "battle buses" and stump speeches, setting out starkly opposed visions of the potential economic impact of a British exit.

Vote Leave said quitting the EU — a prospect known as Brexit — would save Britain billions which could be used to fund the overstretched National Health Service. Justice Secretary Michael Gove said the U.K. pays the EU 50 million pounds ($71 million) a day, money that could be spent on "British people's priorities."

Opponents say that figure is misleading, since it does not include money Britain gets back from the Brussels-based bloc.

"Remain" campaigners say leaving the bloc and its free-trade zone would severely harm the British economy. The International Monetary Fund warned this week that Brexit "could pose major challenges for both the United Kingdom and the rest of Europe."

Former Treasury chief Alistair Darling told a pro-EU event that there is "nothing patriotic about turning a blind eye to credible warnings of economic disaster."

"When the IMF single us out as facing what will be a self-inflicted wound, we can't ignore it," he said.

Unofficial referendum campaigning has been going on since Prime Minister David Cameron secured an agreement on revised EU membership terms for Britain in February. He says that deal means the U.K. should stay in.

During the official campaign period, Britain Stronger in Europe and Vote Leave will each get free TV ad slots, receive some public funding and be able to spend up to 7 million pounds ($10 million) of their own money.