The unit of Lone Star, LSF XI Investments, made an offer of 176 pence per share on May 20, its third for the British company. The proposal was unanimously rejected by Senior the following day.

The proposal price represented a premium of about 72% of Senior's stock close on May 19.

"The Board of Senior considered the proposal, together with its advisers, and concluded that it fundamentally undervalued Senior and its future prospects," the company said in a statement.

Senior, which supplies parts such as airframes and engine build-up tubes to planemakers, is the latest target of rising private equity interest in British companies.

Shares in the London-listed firm jumped as much as 39% to 164.1 pence, their highest since the pre-pandemic levels of February 2020.

Lone Star has until June 25 to make a firm offer or walk away from the deal.

Senior's business has been hit hard by the pandemic, with aerospace sales declining 25% in the first quarter of 2021.

($1 = 0.7062 pounds)

(Reporting by Yadarisa Shabong in Bengaluru; Editing by Aditya Soni)