ValueAct has built its stake from less than seven per cent to more than eight per cent during April, having stayed under the radar for months with a smaller holding.
The hedge fund specialises in taking stakes in firms it believes to be undervalued before pushing for change to improve returns. It was part of an unsuccessful bid for banking software group Misys last year.
Invensys attracted takeover interest from several groups including Emerson Electric last summer, but the talks fizzled out.
Siemens agreed to buy Invensys’ rail unit for £1.7bn in November, sending the group’s shares soaring by a third as traders eyed the chance of a new bid for the entire firm. The disposal of the rail unit, which brought in 40 per cent of group profits, allowed Invensys to pay down some of its pension deficit and hand back £625m to investors.
Invensys declined to comment yesterday, while ValueAct could not be reached for comment.