UPDATE 1-Electra says criticism from activist Bramson "beggars belief"

(In first paragraph, corrects Yates's position to Chairman)

* Electra chairman criticizes Bramson's track record

* Dismisses claims of underperformance

* Showdown set for general meeting on Oct. 6

LONDON, Sept 22 (Reuters) - The chairman of British private equity firm Electra has hit back at Edward Bramson, saying the activist investor's criticism of its investment record "beggars belief" and questioning his revamp proposals.

Bramson's investment vehicle Sherborne said in a letter to Electra shareholders last week that a reorganisation of the company could create more than 1 billion pounds ($1.6 billion) of shareholder value, effectively doubling the company's market capitalization.

But Electra, which traces its origins back to 1935 and whose holdings range from holiday parks operator Park Resorts to fine art materials supplier Daler-Rowney, called into question the proposals, saying they lacked detail and criticizing Bramson's track record.

"When someone sets out their stall as being some kind of operational messiah, we feel the need to examine that," Electra Chairman Roger Yates said in a telephone interview with Reuters on Monday, taking particular issue with Bramson's call for more people with private equity experience on the board.

"We think we have the right balance on the board. He says we have insufficient operational expertise - I personally take that as a bit of an insult," Yates said.

Yates also cited Bramson's position in F&C Asset Management, saying that despite a sharp rise in share price the firm had underperformed its peers during his tenure. Bramson led a boardroom coup at the company in 2011.

Sherborne declined to comment.

PASSIVE INVESTORS

Media-shy Bramson, who has a history of taking on the management of companies he considers to be underperforming, also took a stake in private equity fund 3i Group Plc last year, selling out in December. However he remained a passive investor and never approached management.

Both Electra and Sherborne will be meeting shareholders over the next few weeks in attempts to woo support ahead of an Oct. 6 general meeting forced by Bramson.

In its letter, Sherborne criticized Electra's fees and compared its performance unfavourably with the FTSE 250 index of mid-range stocks, rather than the FTSE All-Share (Frankfurt: 5CI.F - news) index typically used by management.

But Electra said its fees were in line with the market and it had delivered an annualised return on equity of 14 percent over the 10 years to the end of March 2014, hitting its target of between 10 and 15 percent.

It added that it had outperformed the Morningstar (NasdaqGS: MORN - news) index of listed private equity funds and was in the top third of unlisted funds with a similar mandate from the 2006 vintage.

"How you characterize that as unsuccessful really beggars belief," Yates said.

Yates acknowledged Sherborne's criticism of Electra's borrowing costs, saying there had been some "cash drag" on performance, but that the debt was justified given the financial environment of the time.

Electra shares were trading up 0.8 percent at 2,686 pence by 0946 GMT. The stock hit a record high of 2,785p in early March in the wake of Sherborne's stake first becoming public.

($1 = 0.6121 British Pounds) (Editing by Mark Potter and David Holmes)