Activist hedge fund Elliott Management has escalated its campaign at SSE, criticizing the U.K. energy group for refusing to spin off its renewables unit and calling for a refreshed board to analyze options the activist believes would unlock 5 billion pounds ($6.6 billion) of value.

Three weeks ago, SSE outlined plans to invest a bumper 12.5 billion pounds over the next five years to fund a shift to renewable energy in a push against Elliott’s call for a breakup. SSE said it would sell 25% stakes in its electricity transmission and distribution networks and cut its dividend by around 30% from 2024 to fund the green transition.

Elliott, a top-five investor in SSE, sent a letter Tuesday to SSE Chairman John Manzoni saying the company’s rationale to keep its renewables division failed to convince markets and that the plan to sell just a quarter of networks lacks “ambition.”

Elliott noted that SSE’s stock fell 4% the day it unveiled its new green strategy on November 17 and urged the energy company to “provide a detailed and credible plan to address investor concerns around SSE’s corporate governance, its ability to fund its growth in the long term, and its persistent undervaluation.”

The activist wants SSE to add two new independent directors with renewables expertise to the board and create a strategic review committee. The new body would analyze various steps to create value for shareholders such as a partial listing of its renewables arm and potential sale of a minority stake in the unit.

Elliott argued that keeping the newly listed renewables business under control would avoid the potential dis-synergies cited by management when deciding against the breakup. A partial listing would enable the renewables division to “re-rate to a higher valuation and raise equity on capital markets at attractive terms in order to accelerate its growth,” argued Elliott senior portfolio managers Jeff Rosenbaum and Nabeel Bhanji.

The hedge fund also argued for a more ambitious disposal of SSE’s networks business, saying recent transactions in the sector suggest a bigger deal would attract a higher valuation.

“We believe that—with the right steps—there is a clear path for the company [SSE] to unlock £5 billion of untapped value and establish its leadership position as the U.K.’s renewables champion,” Elliott’s Rosenbaum and Bhanji said.

Shares in SSE were slightly down at 16.25 pence each as of 9:30 a.m. Greenwich Mean Time Tuesday, giving the Scottish energy company a market value of 17.3 billion pounds ($23 billion).