UPDATE 3-Securitas to buy Stanley Black & Decker's electronic security business

* Securitas' biggest acquisition to date

* Securitas bets on electronic surveillance for growth

* Deal to be finalised in the first half of 2022

* Stanley Black & Decker to use funds to repurchase shares (Adds Stanley Black & Decker comment, background, share reaction)

STOCKHOLM, Dec 8 (Reuters) - Sweden's Securitas said on Wednesday it had agreed to buy U.S. rival Stanley Black & Decker's electronic security solutions business for $3.2 billion in its biggest acquisition to date.

Securitas, the world's biggest listed security services group, has been pushing for years to increase sales of electronic security services and reduce its exposure to staff-intensive guarding. The firm's biggest cost is paying salaries for its 355,000 employees.

"The future of security is built around the combination of global presence, connected technology and intelligent use of data and, together with Stanley Security, Securitas is perfectly placed to win in this environment," it said in a statement.

With the deal, Securitas will buy back a business it originally floated on the Stockholm bourse in 2006 under the name of Securitas Systems. Stanley Black & Decker bought it in 2011.

Securitas said it expected to complete the deal in the first half of 2022 with acquisition-related costs totalling about $135 million, the majority of which would be booked in 2022 and 2023.

"The acquisition has significant commercial synergies and creates compelling cost synergies which are expected to be fully realised within three years from completion," it said.

It said it would fund the deal through an underwritten bridge facility and a $915 million equity rights issue intended to be launched following completion of the deal.

Current shareholders had provided commitments, declarations of intent and guarantees to subscribe for 44.6% of the rights issue, it said.

Shares in Securitas, whose services include guards, alarm surveillance and airport security and whose biggest competitor is privately held U.S. group Allied Universal after it bought G4S this year, were up 2.6% in mid-morning trade.

Stanley Black & Decker said the sale was part of a strategy to sharpen its focus on core businesses and said it would use proceeds to partly fund a $4 billion share repurchase in 2022.

(Reporting by Anna Ringstrom; Editing by Helena Soderpalm, Louise Heavens and Edmund Blair)