* Firms agreed 11 billion pound merger in March
* 800 jobs to go once deal completed
* Partners to take 320 million pound charge (Adds detail from statement, analyst reaction)
By Carolyn Cohn and Simon Jessop
LONDON, May 10 (Reuters) - Standard Life and Aberdeen Asset Management expect to cut 800 jobs, nearly 10 percent of the firms' combined workforce, as part of a merger to create Britain's biggest listed investment manager.
The companies said they planned to take a one-off charge of 320 million pounds ($415 million) to help pay for the plan.
The cuts form a key part of the Scottish companies' agreed 11 billion pound ($14.2 billion) all-share deal, announced in March, which will give the firm combined assets of 670 billion pounds and aims to save 200 million pounds in annual costs.
In a merger prospectus released late on Tuesday, Standard Life said 75 percent of those annual cost savings would be seen by the end of the second year of the combined company, to be renamed Standard Life Aberdeen.
The firms are looking to shore up their defences in the face of competition from lower-cost index funds and the increased cost of tougher regulations - pressures that are expected to prompt more industry consolidation.
While savings would offer a potential short-term earnings uplift, Barclays analyst Alan Devlin said he remained 'underweight' on both companies' stocks, in a note to clients.
"In our view the merger will do little to improve underlying outflows in each businesses' key asset management franchise."
Standard Life also gave a trading update, saying it had "made further progress" in the first three months of 2017, with net inflows of 3.1 billion pounds across its products.
While that helped take the group's total assets under administration to 361.7 billion pounds, it excluded outflows of 2.8 billion pounds from its flagship Global Absolute Return Strategies (GARS), continuing a period of weak performance.
"Net flows for SL were disappointing," said Bernstein analyst Edward Houghton in a note to clients.
"On balance we'd expect "The Street" could bring numbers down somewhat for (a) standalone Standard Life, though we note that Aberdeen posted a better-than-consensus first-half result last week."
Aberdeen's half-year results on May 2 showed a jump in revenues and profits, and a slowdown in the pace of outflows from its emerging market equities funds.
Shares in Standard Life and Aberdeen were up 0.2 percent at 0744 GMT, in line with the broader market.
Despite planning to shed jobs, the board of the combined firm is set to grow in size to 16 members, equally split between both firms, despite Aberdeen shareholders holding just a third of the new company.
Headed by Gerry Grimstone, current Standard Life chairman, Standard Life and Aberdeen chief executives Keith Skeoch and Martin Gilbert will be co-chief executives of the new firm, a structure some investors have criticised.
Despite that, Aberdeen's largest shareholders - Mitsubishi UFJ Trust and Banking Corporation and Lloyds Banking Group , with which Aberdeen has strategic relationships - have both pledged their support for the deal.
Shareholders of both firms will vote on the merger at extraordinary general meetings on June 19, with three quarters of Aberdeen shareholders needed to approve the deal for it to proceed. The deal is expected to complete by mid-August. ($1 = 0.7733 pounds) (Additional reporting by Subrat Patnaik in Bengaluru; Editing by Keith Weir)