Britain's FTSE index set to post fourth consecutive month of gains

* FTSE 100 index down 0.1 percent

* Set to post fourth straight month of gains

* Hits one-month high before easing

* Alliance Trust rises on M&A news (Adds detail and updates prices)

By Kit Rees and Atul Prakash

LONDON, May 31 (Reuters) - Britain's top share index was setto post its fourth straight month of gains on Tuesday, thoughmining companies lost ground after metals prices came underpressure from a rise in the dollar.

The UK mining index fell 1 percent as copperprices declined on a resurgent dollar following increasingexpectations of a U.S. interest rate hike.

Copper prices remained on track for a more than 7 percentdecline in May, the biggest monthly drop since November.

St. Louis Federal Reserve President James Bullard on Mondayechoed remarks by U.S. central bank chief Janet Yellen thatglobal markets appear to be "well-prepared" for a summerinterest rate hike by the Fed, although he did not specify adate for the move. A stronger dollar makes commodities moreexpensive for holders of other currencies.

Shares in BHP Billiton, Antofagasta,Glencore and Rio Tinto fell 1 to 2 percent,dragging the blue-chip FTSE 100 index 0.1 percent lowerto 6,263.84 points by 1115 GMT.

Oil companies BP and Royal Dutch Shell alsofell, down 1.6 percent and 0.5 percent respectively as the priceof Brent crude eased on rising output from the Middle East andahead of an OPEC meeting later this week.

However, the commodity-heavy index touched a one-month highof 6,290.07 points earlier in the session. It has remained inpositive territory this month and was headed for its fourthstraight month of gains.

"The FTSE 100 is still in the midst of sidewaysconsolidation, but with a welcome test of last week's 6,280ceiling and a close venture to 6,300," said Augustin Eden,analyst at Accendo Markets, adding that a drop towards 6,240would send a bearish signal for the index.

Among mid-caps, Alliance Trust rose 3.2 percentafter RIT Capital Partners made an informal mergerproposal for the investment firm. RIT confirmed talks saying amerger possibility was at "a very preliminary stage ofconsideration".

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If you have any thoughts, suggestions orfeedback on this, please email mike.dolan@thomsonreuters.com.

Mike Dolan, Markets Editor EMEA. (Reporting by Atul Prakash; Editing by Richard Balmforth)