He made the statement in an open letter to GAM shareholders on Friday.

The Swiss firm agreed to be taken over by British rival Liontrust earlier this month in an estimated £96m deal, with the acquisition expected to complete in the final quarter of this year.

But Mr Garzetti said his investment firm, Taure Invest AG, would invest £58m in GAM Holding in an alternative plan to Liontrust’s offer.

He said the British firm’s offer undervalued GAM, would lead to its breakup and involved "significant execution risks."

He wrote: “I am convinced GAM will continue to have many advantages as an independent Swiss asset management company in the future.”

After the letter was published on Friday, GAM confirmed that it had rejected Mr Garzetti’s offer, first made on 3 May, “because it materially undervalued the firm.”

GAM said its board strongly recommended the Liontrust offer to shareholders.

David Jacob, chairman of GAM Holding AG, said: “The offer from Liontrust has been well received by clients and is strongly supported by GAM’s portfolio managers which enables GAM to move forward with stability as part of a combined business with Liontrust.”

“The Liontrust offer was deemed by the board to be the best for all stakeholders. The resulting business will have a strong balance sheet, a broader array of excellent investment products, and a global distribution footprint from which to deliver growth, in which our shareholders can participate in the future”.

GAM has had a turbulent few years, hit by a falling share price and customer outflows.

It is headquartered in Zurich, employs 594 people in 14 countries and has UK investment centres in London and Cambridge.

Liontrust plans to merge the investment management businesses which will have 12 funds with assets more than £1bn (seven managed by Liontrust and five by GAM).

The fund manager intends to rebrand all GAM funds as Liontrust.