Britain's FTSE closes off highs after new referendum poll

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* FTSE 100 ends 0.6 pct higher

* Miners among top gainers

* AB Foods falls on downgrade

* Debenhams (Other OTC: DBHSF - news) drops after results

By Atul Prakash

LONDON, June 22 (Reuters) - Britain's main share index hit a two-week high on the eve of Thursday's vote on EU membership before surrendering half the day's gains in the last minutes of trading after a poll gave the edge to the Leave campaign.

According to a survey published by polling firm Opinium around the market close, the campaign for Britain to leave the European Union held a one-point lead over the Remain camp before Thursday's membership referendum. Its previous poll on Saturday had put the two sides level at 44 percent.

The blue-chip FTSE 100 index closed 0.6 percent higher at 6,261.19 points after earlier rising as far as 6,315.62, the highest level since June 7. The index finished higher for a fourth straight day and is up nearly 4 percent so far this week.

"Markets seem content to trot higher ... The confidence displayed in financial markets is at odds with the uncertainty portrayed by opinion polls. With (Other OTC: WWTH - news) only a day to go, investors are using every available bit of information at their disposal," Jasper Lawler, analyst at CMC Markets (LSE: CMCX.L - news) .

The market was helped by a rally in mining stocks after copper prices rose to their highest in more than two weeks on a weaker dollar, though traders said a lack of liquidity in the metals market meant moves were exaggerated.

The UK mining index rose 1.1 percent as shares of Glencore (Xetra: A1JAGV - news) , Anglo American (LSE: AAL.L - news) and Rio Tinto (LSE: RIO.L - news) all advanced 1.8 percent.

Among other top gainers, housebuilders Taylor Wimpey (LSE: TW.L - news) , Persimmon (Other OTC: PSMMF - news) , Berkeley Group and Barratt Developments rose between 1 and 2 percent. Insurers also rallied, with Direct Line (Other OTC: DIISD - news) and RSA Insurance (LSE: RSA.L - news) rising more than 2 percent.

On the downside, AB Foods fell 3.4 percent, the top faller on the blue chip index, as Morgan Stanley (Xetra: 885836 - news) cut its rating to "equal weight", taking a more cautious view on valuation as it examined why Primark's like-for-like growth was slowing.

Among mid caps, department store group Debenhams fell 4.6 percent after posting a fall in third quarter sales growth as trading conditions became more uncertain, especially in clothing.

"Debenhams has become the latest casualty of poor conditions in the UK clothing market, with falling sales showing how difficult it is for retailers on the high street," George Salmon, equity analyst at Hargreaves Lansdown (LSE: HL.L - news) , said.

"The continued growth of online sales against this backdrop reflects how shopping habits have changed, and how even high street retailers need to be on top of their game when it comes to their online presence." (Additional reporting by Kit Rees; Editing by Ruth Pitchford)