The founder and head of Osborne Clarke's Belgian tax practice is leaving the firm to start his own tax boutique.

Laurent Donnay de Casteau announced the new firm, named ADVISIUS, in a post on LinkedIn. He leaves with Nawel Benaisa, a junior tax associate. Lionel Wellekens, formerly senior manager for indirect taxes at Deloitte Belgium, joins as counsel, according to the post.

"We wish him all the best," Thierry Viérin, Osborne Clarke's managing partner for Belgium, said in a telephone interview. Donnay de Casteau will leave at the end of the month, Viérin said.

Donnay de Casteau founded the firm's tax practice in Belgium when he joined the firm in 2015. While at Osborne Clarke, he specialized in all aspects of corporate and personal tax, with extensive experience in carrying out international transactions. His practice focused on corporate structuring as well as restructuring, the tax aspects of mergers and acquisitions, private equity, investment funds, high-net-worth individuals and expat issues, according to a cached version of his official web page on the firm's site. His page has been removed.

Before joining Osborne Clarke, Donnay de Casteau spent over 15 years working at Loyens & Loeff, a top-tier Benelux law firm.

Olivier Lambillon, currently counsel, will become the new head of tax in Osborne Clarke's Brussels office, Viérin said. Lambillon's expertise lies in various areas of tax law, including direct taxation, indirect and personal tax issues. He joined Osborne Clarke in September 2016.

In his LinkedIn post, Donnay de Casteau said that because ADVISIUS is independent, the team is able to serve clients in the most flexible way. The company will assist clients by coordinating the intervention of other specialists, and by intervening in specific tax questions together with clients' teams.

Donnay de Casteau could not immediately be reached for comment.

Simon Lock contributed to this report.