UPDATE 3-Keysight to launch an offer for London-listed Spirent, source says

(Changes sourcing)

March 27 (Reuters) - U.S.-based Keysight Technologies is preparing to launch a formal offer for British telecommunications testing firm Spirent Communications, a person familiar with the matter told Reuters on Wednesday.

Sky News reported the development first and said the offer would be valued at about 1.15 billion pounds ($1.45 billion), or roughly 200 pence per share.

On March 5, U.S.-listed communications equipment firm Viavi Solutions had agreed to buy Spirent for about 1.01 billion pounds. As part of the deal, Spirent shareholders would get 175 pence per share.

The offer from Keysight represents an 84.5% premium to Spirent's closing price of 108.40 pence on March 4, compared with Viavi's premium of 61.4%.

Some institutional shareholders in Spirent had indicated that they would vote in favor of Viavi's offer, but believed it was opportunistic and left the door ajar for a rival bidder, the Sky News report added.

Spirent Communications, Viavi Solutions and Keysight Technologies did not immediately respond to Reuters requests for comment. ($1 = 0.7915 pounds) (Reporting by Shivani Tanna and Aishwarya Jain in Bengaluru and Anousha Sakoui in London; Editing by Vijay Kishore and Devika Syamnath)