Minerva, the London developer, today urged shareholders to reject an £84.5m offer from South African entrepreneur Nathan Kirsh, claiming it was “opportunistic” and “wholly inadequate”.

Minerva's response circular to shareholders showed its portfolio was valued at around £1bn at the end of November, lifting its net asset value to 95p a share, nearly double the 50p-a-share bid.

The £1bn valuation from CB Richard Ellis reflected net growth of £93m since the end of June, equivalent to an increase of 10% in five months.

Minerva’s shares were up 12.7% to 62p at 11.00am, valuing the company at £100m.

“What is particularly opportunistic about timing of this bid is that it comes after we have done the hard yards and just as we are beginning to see the turn,” chief executive Salmaan Hasan told a conference call.

“We are not going to give away Minerva having done all the work and seriously reduced the risk just as the market improves in the shareholders' favour.”

Kirsh and entities related to his KiFin vehicle own 29.9% of Minerva’s shares.

Minerva's two major City of London office developments, The Walbrook and St Botolphs, represent half the total office development pipeline set to complete in the financial district by the end of 2010.

Hasan said the company was in “serious” talks with a number of parties interested in taking space at the schemes.