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* FTSE 100 down 0.3 pct
* Miners drag index lower
* Standard Life at 15-month high on Aberdeen merger
* Ultra Electronics Holding top STOXX gainer after results
* Acacia Mining extends losses on Tanzania export ban
LONDON, March 6 (Reuters) - Britain's blue-chip FTSE 100 index edged down on Monday as weakness in mining stocks outweighed the positive impact of a potential 11 billion pound merger between Scottish fund managers Standard Life and Aberdeen Asset Management.
The index was down nearly 0.4 percent at 1014 GMT, tracking losses in the broader European index.
Standard Life was the top gainer, up 5.4 percent after hitting a 15-month high in early trading on news of a merger with mid-cap peer Aberdeen Asset Management. Aberdeen shares were up 4.3 percent.
The funds, seeking synergies and cost savings to fight back against cheap, index-tracking funds, said they expected to save 200 million pounds through the merger. The deadline for the deal is April 1.
"The deal makes perfect sense as a defensive play. The explosive growth in passive investing trends has heaped pressure on active managers like Aberdeen and Standard Life and consolidation had to be on the cards," said Neil Wilson of ETX Capital.
Shore Capital warned of "grave concerns" over the proposed board structure of the combined company, particularly the appointment of co-CEOs.
Anglo American, Glencore, Rio Tinto, Fresnillo and Antofagasta were all top fallers on the blue-chip index, down 1.4 to 1.9 percent in early trade as copper prices slipped due to risk-off sentiment in broader markets.
Mid-cap miners Kaz Minerals and Vedanta Resources also fell 2.5 to 3.7 percent.
EasyJet was a top riser, up 2 percent after it said passenger numbers had increased 8.2 percent in February.
International Consolidated Air, the owner of British Airways, gained 1.6 percent.
Strong gains from Aberdeen and Ultra Electronics Holding helped the mid-cap index outperform.
Ultra Electronics Holding hit an all-time high, up 6 percent, the top mid-cap gainer and best-performing on the STOXX 600, after it posted an increase in full-year profit, and order intake up 22 percent.
Ultra sees global defence spending rising around 3 percent this year, according to Edison research analysts.
"We see room for further upgrades should U.S. budget commentary convert into spend over the medium term," said Investec analysts, who rate the company a 'buy'.
"For us, Ultra offers the best value in the UK Aerospace & Defence sector and is deserving of a re-rating."
Acacia Mining was the top mid-cap faller, down 6.2 percent after it dropped 13.5 percent on Friday when the Tanzanian Ministry of Energy and Minerals issued a surprise ban on gold/copper concentrate exports.
Acacia operates three gold mines in Tanzania, accounting for 30 percent of its revenues.
The miner said in January it was in preliminary merger discussions with Canada's Endeavour Mining, but analysts said the export ban could jeopardise a deal.
"With no clarity over a type or timing of a resolution, we fear Acacia shares will be suppressed," said Jefferies analysts, downgrading the stock from 'buy' to 'hold'.
"In our opinion, with a spike in jurisdictional risk in Tanzania, merger discussions with Endeavour are at the very least going to be delayed," said Jefferies.
Reporting by Helen Reid; Editing by Mark Trevelyan