Bloomberg is funding campaign to keep U.K. in Europe

The 'Brexit' debacle, explained
The 'Brexit' debacle, explained

The U.K. debate on whether to remain a member of the European Union is heating up and money is rolling in to support both sides.

The campaigns for and against Britain's exit (Brexit) from the EU revealed the names of their top donors Wednesday. Media group Bloomberg LP is among the biggest backers of the campaign to persuade Brits to stay in.

The referendum takes place on June 23. Here are some of the other top donors so far:

The Ins

Those campaigning for Britain to remain in the EU have raised over £7.3 million ($10.6 million), with the lead campaign group -- Britain Stronger in Europe -- receiving around £6.9 million ($10 million).

Their single biggest backer is David Sainsbury, great grandson of the founder of the J.Sainsbury (JSAIY) supermarket chain and a member of the House of Lords. He has given more than £2.7 million ($3.9 million) to the various "remain" campaigns.

Related: Brexit is already hurting the UK economy

Hedge fund billionaire David Harding gave £750,000 ($1 million) while Lloyd Dorfman, the founder of foreign exchange firm Travelex, added £500,000 ($724,000) to the pot.

Bloomberg LP donated £250,000 ($362,000) via its Bloomberg Tradebook Europe subsidiary. That was matched by private equity firm Grovepoint Capital, Morgan Stanley (MSPRF), Citigroup (C) and steel firm William Cook Holdings.

The Outs

The pro-Brexit campaigns so far have received £8.2 million ($11.8 million) in donations. The lead campaign, Vote Leave, got £2.8 million ($4 million).

Unlike the "remain" camp, the "Brexiteers" don't yet have any big name corporate donors. Their main sponsors are individuals.

Peter Hargreaves, the co-founder of financial services firm Hargreaves Lansdown, donated £3.2 million ($4.6 million) to Leave.EU campaign. Businessman Patrick Barbour gave over £500,000 ($724,000) to Vote Leave.

The rules

The official lead campaign group for each side -- Britain Stronger in Europe and Vote Leave -- have their spending capped at £7 million ($10 million). But other registered groups can campaign on either side, and there is no overall limit on how much can be spent.

Campaign groups are required to report details of any individual loan or donation over £7,500 ($10,860). On top of the donations, both lead campaigns are also entitled to a grant to help with costs such as TV broadcasts and a mailing to voters.

The campaign will run for six more weeks, so more donations could come in.

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