TAIPEI (Reuters) - Taiwanese conglomerate CTBC Financial late on Friday raised its bid for Shin Kong to a 51% stake of the company, challenging an existing plan for its smaller rival to merge with Taishin.

Updating a previous announcement from Tuesday, CTBC said it had upped its purchase plans from a 25% stake, at a price of T$14.55 per share.

CTBC said it plans to submit its plan to regulators early next week.

Shin Kong, whose stock closed flat at T$12.45 on Friday, did not immediately respond to a request for comment.

Shin Kong and Taishin said on Thursday they planned to merge, a long mooted proposal first considered two decades ago.

The merger would happen through a stock swap, the companies said in joint statements.

Shin Kong and Taishin first held merger talks in 2002 and speculation has swirled since about the two coming together.

(Reporting by Emily Chan and Ben Blanchard; editing by Sam Holmes)