* FTSE 100 down 0.4 pct
* Britons go to the polls in test of Prime Minister May
* Laundry deal boosts Berendsen
* Ex-divs weigh (Recasts, adds detail, updates prices at close)
By Helen Reid and Kit Rees
LONDON, June 8 (Reuters) - The UK's top share index dropped to a three-week low on Thursday as Britons headed to the polls after a tumultuous campaign which saw Prime Minister Theresa May's lead tighten in recent weeks.
The major FTSE 100 benchmark slid steadily throughout the session to close 0.4 percent lower at 7,449.98 points, after the European Central Bank signalled an end to more interest rate cuts, and ahead of results from Britain's parliamentary elections.
Polls indicated Prime Minister May's gamble on securing a bigger Conservative majority would be rewarded with a win, though the surprise 2016 vote to quit the European Union meant investors were considering all scenarios.
Mid-caps have outperformed the exporter-heavy FTSE 100 so far this year but the blue-chips have gained back some ground in recent weeks as tightening polls prompted greater caution on domestic stocks.
"Some investors, possibly the market makers, have been told to get their books fairly straight rather than having long positions in stocks that could take a caning," said Paul Mumford, manager of the Cavendish opportunities fund.
"If you were to get a coalition, or Labour win, or a very small, difficult-to-work Conservative result, then the market would take it quite badly in the short term, the pound would drop and this would provide a boost to the exporters," he added.
"So the FTSE 100 has performed relatively well because people want to have an overseas hedge."
Inversely, a bigger majority for the Conservatives could prompt institutions to pick up some bargains among mid-caps, Mumford said.
U.S. bank Citi advised investors to hedge political risk through the "UK 8", a list of blue-chip stocks least exposed to domestic sales, while avoiding the "Anti UK 8" stocks with the greatest dependence on the British economy.
Traders in the City of London prepared for a long night, with banks and brokers pulling in extra staff to cover for potential volatility as results trickle in.
While macroeconomic events were front and centre of investors' minds, some company moves stood out.
Centrica and SSE were among the top 10 blue-chip gainers. Their share prices have dipped in recent weeks as the election approached, with investors concerned about manifesto promises to cap energy prices or nationalise the companies.
Firms going ex-dividend weighed, however, with Vodafone falling 4.8 percent and WPP down 2.7 percent.
Among mid-caps, Berendsen jumped 11 percent, the most actively traded stock as investors cheered a merger with Elis after the French laundry firm sweetened its offer for the company.
Elis' latest offer valued Berendsen at 2.2 billion pounds, or 1,250p a share, a 45 percent premium to its closing price before the initial takeover offer last month.
Petrofac shares jumped more than 4 percent, among top European gainers, after the oil services firm sealed a 10-year deal with Petroleum Development Oman. (Reporting by Helen Reid and Kit Rees; Editing by Jon Boyle and Toby Davis)