Bramson's Sherborne Investors, which has built up a stake of around 19 percent in Electra since February, on Wednesday promised to add more than 1 billion pounds to Electra's market value to take its share price to around 6,000 pence ($98).

Shares in Electra were trading at 2,729 pence at 1402 GMT (03:02 p.m. BST), up 0.04 percent on the day.

"We note Sherborne's letter issued this morning and will be giving a forceful response in due course," an Electra spokesman said in a statement.

Sherborne is the company's biggest shareholder and has forced a general meeting, scheduled to take place on Oct. 6.

It will be meeting other investors on an individual basis before then, asking them to elect Bramson and former Sherborne Chairman Ian Brindle to the board.

Sherborne is also requesting former Bain director Geoffrey Cullinan be removed from the six-strong board of non-executive directors. Electra rejected a request by Bramson to join the board in July.

In the letter, Sherborne criticises Electra's high debt costs and the performance of its more recent investments, and requesting strategic and operational changes. It did not go into depth on what changes it was seeking.

The letter also compares the performance of Electra, which takes stakes in small to mid-market companies, unfavourably with the FTSE 250 <.FTMC>, as opposed to the FTSE All-Share <.FTAS> which has been used as a comparison by the fund.

"The 10-year return of the more comparable FTSE 250 index was almost double that of the FTSE All-Share, so comparisons with the latter, of course, present performance in a more flattering, but less relevant, light," Sherborne said.

Analysts from JP Morgan Cazenove said in a note: "Sherborne criticises the Board for lack of private equity experience, but ironically wants to remove the founder of Bain's private equity business in Europe. We remain Neutral."

Sherborne declined to comment.

(Reporting By Freya Berry; Editing by Pravin Char)