Witan is to consolidate five of its investment portfolios into three to try to improve the performance of the funds.
The move means that accounts managed by Browne Company and MFS Investment Management and Tweedy will both be closed.
Browne managed around 3 per cent of Witan’s total assets, while MFS managed 5 per cent.
The three remaining portfolios will be managed by Lansdowne Partners, Pzena Investment Management and Veritas Asset Management.
Each of the three firms oversee around 14.3 per cent of Witan's overall assets.
Andrew Bell, Witan's chief executive, said he hoped the changes would increase the performance potential of the overall combined portfolio.
“Although manager risk diversification is one of the benefits of the multi-manager approach, there is a balance to be struck.”
He said this balance was between reducing performance volatility and having a structure with the potential to deliver attractive levels of outperformance.
The transition means that Witan's combined active share is expected to rise to 74 per cent from 70 per cent at the end of last year.
Following this change, Witan will have nine external managers covering UK, European, Asian, Emerging Market and Global markets.
Witan’s executive team also invests up to 10 percent of the assets in a range of collective funds covering specialist asset classes.