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 set to post its fourth straight month of gains on Tuesday, though mining companies lost ground after metals prices came under pressure from a rise in the dollar.

The UK mining index fell 1 percent as copper prices declined on a resurgent dollar following increasing expectations of a U.S (Other OTC: UBGXF - news) . interest rate hike.

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

St. Louis Federal Reserve President James Bullard on Monday echoed remarks by U.S. central bank chief Janet Yellen that global markets appear to be "well-prepared" for a summer interest rate hike by the Fed, although he did not specify a date for the move. A stronger dollar makes commodities more expensive for holders of other currencies.

Shares (Berlin: DI6.BE - news) in BHP Billiton (NYSE: BBL - news) , Antofagasta (Other OTC: ANFGF - news) , Glencore (Xetra: A1JAGV - news) and Rio Tinto (LSE: RIO.L - news) fell 1 to 2 percent, dragging the blue-chip FTSE 100 index 0.1 percent lower to 6,263.84 points by 1115 GMT.

Oil companies BP and Royal Dutch Shell (Xetra: A0ET6Q - news) also fell, down 1.6 percent and 0.5 percent respectively as the price of Brent crude eased on rising output from the Middle East and ahead of an OPEC meeting later this week.

However, the commodity-heavy index touched a one-month high of 6,290.07 points earlier in the session. It (Other OTC: ITGL - news) has remained in positive territory this month and was headed for its fourth straight month of gains.

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

Among mid-caps, Alliance Trust rose 3.2 percent after RIT Capital Partners (LSE: RCP.L - news) made an informal merger proposal for the investment firm. RIT confirmed talks saying a merger possibility was at "a very preliminary stage of consideration".

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