A British vote to leave the EU would send shockwaves through financial markets and an overwhelming majority of economists polled by Reuters said a so-called Brexit posed the biggest threat to UK growth.

Support for leaving and remaining are still running neck-and-neck, opinion polls show.

Europe's biggest insurer, Allianz, on Wednesday said it was pursuing asset-liability matching for its insurance business in Britain and maintains no open exposure to sterling.

"We keep our obligations and our investments almost entirely currency matched, so we therefore have made sure that we have no open positions against the pound," Chief Financial Officer Dieter Wemmer told a conference call with journalists on the insurer's first-quarter earnings.

"Otherwise, I'd say you can't prepare (for Brexit) because if the outcome is 50/50, it's a pure bet on which side to prepare for, it's like a tossing a coin."

The European Central Bank is looking closely at lenders' plans to deal with the potential impact but the insurance business is organised and supervised on a national basis and would remain so after the vote.

"We have not placed a bet for either outcome," said Roland Vogel, Chief Financial Officer of reinsurer Hannover Re, which has about 8 percent of its 40 billion euros (31 billion pounds) of investments and obligations denominated in pounds.

The world's third-largest reinsurer has run calculations on the impact of a Brexit, Vogel said.

"The British pound would certainly lose value relative to the euro but that is always a two-sided game because the obligations would also become cheaper," Vogel said.

"We are and will stay invested in British pounds," he added.

Peer Munich Re said London's importance as a reinsurance market would be diminished by a vote to leave the EU, potentially benefiting other centres, such as Singapore.

Insurers do see an exit as creating broader economic and political damage both to the UK and the EU, particularly if negotiations to establish a new relationship were to drag on, leading to economic uncertainty and volatility.

Allianz's Wemmer said a 'leave' vote would also entail technical headaches for asset management operations that may need to legally separate mutual funds domiciled in the UK, but said the impact on the business itself would be less dramatic.

(Additional reporting by Arno Schuetze and Alexander Huebner; Editing by Alexandra Hudson)

By Jonathan Gould