U.S. activist ValueAct takes stake in Rolls-Royce

* Hedge fund now biggest shareholder with 5.44 stake

* Follows profit warnings and share slide at Rolls

* Rolls says 'looks forward to engaging' with fund

By Simon Jessop and Svea Herbst-Bayliss

LONDON/BOSTON, July 31 (Reuters) - U.S. activist investor ValueAct has taken a 5.44 percent stake in British engine maker Rolls-Royce to become the biggest shareholder in the company, raising further pressure on the embattled company.

ValueAct crossed the 5 percent threshold at which it needed to alert the company on July 29, a regulatory filing on Friday showed. Rolls-Royce shares jumped 6 percent.

The announcement comes a day after Rolls-Royce reported a 32 percent drop in half-year profits, after previously slashing forecasts three times and warning investors that profits at its aero unit would take a hit next year.

A spokesman for Rolls-Royce said the increase in ValueAct's holding meant it was now the British firm's biggest investor.

"ValueAct has been an investor before and we constructively engaged with them before," the spokesman said, adding in a statement that the company welcomed "any investor who recognises the long-term value of our business".

"We have frequent communication with all of our shareholders and meet with major investors on a regular basis ... (and) look forward to engaging with ValueAct, just as we do with all investors."

A call to San Francisco-based ValueAct was not immediately returned. (Reporting by Simon Jessop; Editing by David Holmes)