June 21 (Reuters) - Lone Star Global said on Monday it had sweetened its offer to takeover Britain"s Senior Plc (SNR.L) to 838.8 million pounds ($1.2 billion), after the aircraft parts supplier rejected a previously undisclosed proposal last week. The U.S.-based private-equity firm said its 200 pence per share offer would be its fifth and final for Senior. It said it could increase the proposal if another party tabled an offer and created a possible bidding war. Shares of Senior, which supplies parts such as airframes and engine build-up tubes to planemakers, rose as much as 22% to 185 pence by 0933 GMT. They have pared losses since Lone Star"s overtures were made public but are still down 12% from the end of 2019. Senior did not have an immediate comment. Through its unit LSF XI Investments, Lone Star had proposed 185 pence per share on June 14, which was also turned down by Senior. It had until June 25 to make a firm offer or walk away. The latest offer from Lone Star was at a premium of about 32% to Senior"s close on Friday. The pandemic hit Senior and other jet suppliers hard and the firm has warned of a challenging 2021 after lingering woes from Boeing"s 737 MAX crisis. Senior had already been restructuring its business by selling non-core units and cutting jobs. read more The London-listed company is one of the latest targets of rising private equity interest in British firms and in the aviation sector. Private jet services firm Signature Aviation was taken private earlier this year. A rebound in air traffic and an increased interest in private jet travel as customers avoid crowded, commercial airlines has drawn interest from private equity funds and infrastructure investors. read more ($1 = 0.7226 pounds) Reporting by Pushkala Aripaka in Bengaluru, Editing by Sherry Jacob-Phillips Our Standards: The Thomson Reuters Trust Principles.