The exodus from investment manager Invesco continued last week with the departure of a third portfolio manager within a month.

Mark Landau, an investment manager in Invesco's small companies team, has followed Rohan Walsh and Luke Sinclair out the door.

Walsh and Sinclair left Invesco's large companies team in March to join Bell Asset Management. Landau will not be joining them and plans to set up his own boutique fund manager instead.

Invesco said Landau's decision to leave was totally unrelated to Walsh and Sinclair. "While I recognise the perception that [Landau's] resignation may create following the recent departures . . . I stress these are unrelated events," Invesco national accounts manager Anna Schofield said.

Landau, who will leave at the end of the month, has only been working on the small companies team since January. He worked alongside small companies head Cynthia Jenkins.

Schofield said Invesco was actively looking for a replacement for Landau.

Invesco's staffing problems have had a negative impact on its ratings and investment mandates. Russell Investment Group this week terminated its mandate with Invesco and will instead split it between two existing managers, Orion and MIR.

Standard and Poor's has placed the four-star rated Australian share fund on hold and research house Morningstar downgraded Invesco's Wholesale Australian Share Fund and Australian Share Fund from 'Recommended' to 'Avoid'.

Another research house, van Eyk, removed Invesco's funds from its Blueprint Series multi-manager fund series, barely two weeks after it had been added.