UPDATE 2-Paddy Power Betfair's shares slump after CEO says to step down

* Replacement named for Breon Corcoran

* Timing of transition yet to be decided

* Corcoran to complete merger inetegration

* Shares fall 8.5 percent (Adds bullets; background on Corcoran)

DUBLIN, Aug 7 (Reuters) - Gambling firm Paddy Power Betfair's chief executive Breon Corcoran announced plans to step down on Monday, 18 months after completing the merger of Irish bookmaker Paddy Power with Britain's Betfair, sending the share price down over 8 percent.

Corcoran, 46, will be replaced by Peter Jackson, 41, a non-executive director at Paddy Power Betfair and CEO of the UK arm of global payments business WorldPay Group since March, the company said in a statement.

Worldpay Group is currently negotiating its own possible sale to U.S. peer Vantiv.

Paddy Power Betfair gave no date for Corcoran's departure, saying only that he would "complete the integration of the Paddy Power and Betfair businesses and ensure the delivery of an orderly transition."

Paddy Power Betfair's share price was down 8.5 percent at 72.50 pounds in London trading by 0830 GMT.

The price is down 17.5 percent since the start of the year following concerns about what the company in May called a "pretty extreme" level of competition in its core European markets and increasing regulatory pressures in Britain and Australia.

"The timing is certainly less than ideal given the particularly noisy regulatory backdrop at present and the fact that the group is yet to complete its major platform migration project," said Davy Stockbrokers analyst David Jennings.

"That said, the fact that the board is in a position to announce Peter Jackson as CEO designate should provide some comfort."

Paddy Power made the announcement after a report by Sky News on Saturday said that the company was searching for a new chief executive.

Paddy Power Betfair Chairman Gary McGann said in a statement the company had "intensified (its) focus on executive succession planning" a few months ago after Corcoran spoke to the board about his long term plans.

The company also announced details of its financial results for the first six months of the year a day early, reporting revenue growth of 9 percent and underlying EBITDA growth of 21 percent and said trading was in line with expectations.

It said it expected underlying earnings before interest, tax, depreciation and amortisation expenses (EBITDA) this year of between 445 million pounds and 465 million pounds ($580-607 million).

The average forecast of six analysts polled by Reuters in recent weeks was 459 million euros.

Corcoran, a former Paddy Power executive who became CEO of Betfair in 2012, was appointed CEO of the group when the merger completed in February 2016. He worked at Paddy Power from 2001 to 2012, latterly as chief operating officer.

Jackson was CEO of Travelex Group from 2010 until its sale in 2015 and was in charge of innovation at Banco Santander before his appointment as CEO of WorldPay's UK arm. ($1 = 0.7661 pounds) (Reporting by Conor Humphries in Dublin and Rahul B in Bengaluru; Editing by Greg Mahlich)