Jeff Ubben’s Inclusive Capital Partners has made a public takeover bid for Countryside Partnerships valued at nearly 1.5 billion pounds.

In a regulatory filing Monday, Inclusive said it had made a previous offer in private but was now going public after the board refused to allow the hedge fund access to its books.

“The In-Cap team believes that Countryside shareholders deserve the opportunity to decide on the merits of any offer, and that if an approach is made in good faith, the Countryside board should act in the interests of its shareholders by engaging with the potential offeror and not deny its shareholders this opportunity,” the ESG activist wrote.

The offer, worth 295 pence per share, would work out at 1.47 billion pounds or $1.86 billion and may allow current shareholders to roll their equity into the deal.

Shares gained more than 18% Monday to close at 283 pence after hitting a high of 311 pence in early trading after Inclusive’s offer was made public. They were slightly up at 284 pence in as of 9:50 a.m. Tuesday in London.

Reacting to the overture, Countryside defended its decision to reject the 295 pence per share offer, and a previously submitted lower one, for “materially” undervaluing its business and prospects. Countryside reckoned its strong position in mixed tenure home building, “significant” cash generation from legacy asset sales, and asset-light partnerships model put it above competitors and make for a “strong platform” to drive its stock price above Inclusive’s bid.

However, the developer’s shares have been falling consistently since August. In January, the company reached a settlement with U.S. hedge fund Browning West to add Peter Lee to its board. CEO Iain McPherson resigned at the same time.

Last month, interim CEO John Martin released a report that identified weak performance in Countryside’s North and Midlands projects, some of which came out of the acquisition of Westleigh Group in 2018. The company also sought to dampen concerns about the rising cost of manufacturing and potential costs associated with enhanced government fire safety standards.

If completed, the deal would be Inclusive’s first-ever buyout. Ubben, the founder of ValueAct Capital Partners, set up his new fund to explore activist opportunities connected with social and environmental impact in public markets.