PARIS (Reuters) - Atos (>> ATOS) is to buy Bull (>> BULL) in an all-French IT sector deal worth 620 million euros ($845 million) to create the top Europe-based cloud computing company and a major player in cybersecurity.

The companies said a combined Atos and Bull, currently numbers 5 and 10 respectively in cloud computing in Western Europe, would leap to the fast-growing industry's number-two spot by revenue behind U.S.-based Amazon (>> Amazon.com, Inc.) and ahead of Microsoft .

A presentation slide on the Atos web site after the pair announced the deal on Monday showed the cloud services market growing at a compound annual rate of between 25 and 50 percent a year.

At 4.90 euros a share, the agreed offer announced by both companies on Monday represents a 30 percent premium to the three-month weighted average share price of Bull and the combination is expected to deliver some 80 million euros annually in cost savings within two years, equivalent to almost a third of Atos' net profit last year.

Analysts at Kepler Cheuvreux said the relatively low multiple for Bull, giving it an enterprise value of 9 times earnings before interest, tax, depreciation and amortisation, reflected its status as a "serial disappointer", while Atos' record on acquisitions was a good one and so "we see some rationale in the transaction".

Bull shares leapt to stand at the offer price on Monday morning, 22 percent higher than Friday's close. Atos shares climbed 4 percent to 63 euros a share.

Atos, more than nine times larger than Bull by market value based on its offer price, said the deal would enhance its offerings in manufacturing, healthcare, and the public sector, and would reinforce its footprint, "mainly in

France, but also in geographies such as Iberia, Poland, Africa

and Brazil".

"Bull will bring critical and complementary capabilities in big data which, combined with Atos solutions, will create a unique offering in this high-growth segment," it said. Bull's main shareholders, Crescendo Industries and Pothar Investments, own some 24.2 percent of the company and have already committed to tender their shares, the companies said.

Atos is also in a battle to acquire French IT services group Steria (>> GROUPE STERIA). Steria has accepted a bid from rival Sopra (>> SOPRA GROUP) and rejected Atos' 22 euro per share proposal.

A spokesman for Atos said the Bull deal did not change its offer for Steria, which remains on the table until Sopra's annual meeting on May 27.

(Editing by Tom Pfeiffer)

By Andrew Callus and Gwénaëlle Barzic