* Raises unit cost forecast for the year
* Output cut by falling ore grades, protests, floods
* Shares fall 1.5 percent (Adds details costs forecast, share price)
By Silvia Antonioli
LONDON, July 29 (Reuters) - Chilean miner Antofagasta cut is annual copper output forecast for the second time this year on Wednesday due to a delay in the start-up of its Antucoya project, as it posted a double-digit-percentage decline in half-year copper production.
The London-listed miner, which has been hurt by a steep fall in prices, declining ore grades, unfavourable weather and environmental protests, produced 303,400 tonnes of copper in the first half, down 12.9 percent from a year earlier and below analysts' forecasts.
It cut its full-year copper production guidance to 665,000 from 695,000 tonnes due to the delayed commissioning and subsequent ramp-up of its Antucoya project stemming from a technical issue.
Torrential downpours in a desert region of northern Chile and environmental protests had already hit its first-quarter output, forcing it to reduce its annual production forecast by about 15,000 tonnes earlier this year.
The company, controlled by Chile's Luksic family, also increased its average cash cost expectations for the year to $1.47 per pound from $1.40 as a result of the reduced output.
"Overall performance during the first half of the year has not been as good as originally expected," Chief Executive Diego Hernández said in a statement.
"Construction of Antucoya was completed on budget but we have experienced some commissioning issues on the crusher circuit which means we now expect first production to be delayed until the end of the third quarter," the company said in a statement.
The firm is focusing on its $1.9 billion Antucoya greenfield project and other brownfield expansions to cope with a fall in production due to aging mines and declining copper grades.
Antofagasta shares were down 1.9 percent by 0749 GMT lagging a 0.6 percent rise in the UK-listed mining sector and making it one of the biggest losers in the FTSE 100 index of UK blue-chip companies. (Editing by Mark Potter and Jason Neely)