The offer has been branded "significantly inadequate," with a price range of between €60m ($69.9m) and €76m said to "neglect" the value of Zeal’s German business.
Zeal believes a sale would strip it of its most valuable asset and the basis for future brokerage growth in Germany.
Dr Helmut Becker, Zeal CEO, said: "The indicative offer from Lottoland is an attempt to buy our core German assets on the cheap. It does not reflect the value of our German business.
"At the same time, a sale of our core business would leave Zeal and its shareholders with all downside risks from pending VAT litigation in Germany and with significant costs from restructuring the rest of the business.
"Lottoland's offer therefore confirms our view that their main intention is to disrupt the Lotto24 transaction, driven by their business interests as a competitor.'"
