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* FTSE 100 up 1.1 pct
* Energy stocks contribute more than half the index's rise
* OPEC to cut supply for the first time since 2008, detail scarce
* Capita slumps after profit warning
By Alistair Smout and Adela Suliman
LONDON, Sept 29 (Reuters) - Britain's top share index rose near a 1-1/2 year high on Thursday, boosted by a rise in its heavyweight energy sector as a decision by OPEC to curb output for the first time since 2008 lifted oil prices.
Energy stocks added 47 points to the FTSE 100, contributing more than half of its rise.
The index was up 75.15 points, or 1.1 percent, at 6,924.53 at 0827 GMT, 0.4 percent away from its 2016 high. That peak, hit in August, was the index's highest level since April 2015.
Heavyweight oil firms such as Royal Dutch Shell and BP were up 5.6 and 4.5 percent respectively.
While Brent crude came off highs overnight, it closed up 6 percent on Wednesday after the Organisation of the Petroleum Exporting Countries struck a deal to limit crude output at its policy meeting in November.
Although details on the deal were scarce, traders were encouraged by the first move from the group to support prices since 2008.
"This tentative move by OPEC could be a game changer and as a result we're seeing oil stocks moving up quite nicely," said Manoj Ladwa, head of trading at TJM Partners.
"At some point they needed to change it up a little bit, there's been an oversupply in crude oil for a number of years now, hence the ultra low prices."
He said that other commodity-related stocks with oil in its portfolio, such as BHP Billiton, would benefit.
BHP Billiton was up 4.9 percent, and other miners also rose, with the FTSE 350 mining index up 3.5 percent.
The stand-out faller on the FTSE 100 was Capita, down 30 percent in its biggest ever one-day fall after a profit warning. It cut its profit outlook for the full-year by as much as 13 percent.
It said that the British vote to leave the European Union had led to delays in its clients' decisions.
"We fear that there may be more negative news to come in due course and place our "hold" stance under review," analysts at Shore Capital said in a note, saying the effects of the uncertainty from the vote could linger for several quarters.
The dramatic drop hit its peers, with Babcock the index's second biggest faller. Among mid-caps, fellow outsourcers Serco, G4S and Mitie were down 1.2-2.6 percent.
Merlin Entertainment fell 4.6 percent, down after it said its major London attractions were struggling despite a fall in sterling after Britain's Brexit referendum in June. (Additional reporting by Kit Rees; Editing by Angus MacSwan)