* Atos made 4.3 bln euro take over bid
* Gemalto calls offer opportunistic and unconvincing
* Shares in both companies fall, Atos by more (Recasts, writes through, updates shares)
PARIS, Dec 14 (Reuters) - Atos will press ahead with its bid for chipmaker Gemalto, despite a rejection of the French technology company's 4.3 billion euro ($5 billion) offer, raising the prospect of a hostile takeover battle.
Franco-Dutch Gemalto called the 46 euro per share Atos offer to create a digital security leader "opportunistic", although its chief executive Philippe Vallee did not completely shut the door on a deal in an interview with French newspaper Les Echos.
Atos, meanwhile, reiterated its readiness to open talks, saying in a statement it was confident Gemalto's board would engage in talks and that it still had "friendly intentions".
Market reaction showed investors were still hoping for an improved bid from Atos, with shares in Gemalto, the world's largest maker of chips found in mobile phones and credit cards, trading at 46.5 euros at 1223 GMT.
"The most likely scenario for us is that Atos raises its offer," Philippe Cohen, fund manager at Kiplink Finance, said.
"Atos started with a low price, which is logical in this type of negotiations. There's also the scenario of a 'white knight', probably French, such as Thales for example."
Shares in Gemalto, which is run out of France but listed in the Netherlands, have surged since news of the Atos bid. They had fallen heavily after four profit warnings in 2017.
Atos shares were 2.54 percent lower at 130.35 euros as investors priced in the likelihood of it having to pay more for its target if it pursues the chase.
Gemalto's SIM cards have equipped millions of phones, but it has been moving away from this slowing business and sought to grab a share of the fast-growing cybersecurity sector instead.
In the past, Gemalto has attracted the attention of governments around the world for its technologies which are key to surveillance apparatus. In 2015, it said U.S. and British spies were likely to have hacked it in an attempt to steal codes that protect the privacy of billions of mobile phone users.
The French government, via its Bpifrance investment bank, is the second-biggest shareholder in Gemalto, behind Germany's Quandt family.
It had said it was favourable to consolidation between two French companies in the tech sector, however officials at Bpifrance were not immediately available for comment on Thursday following Gemalto's rejection of the Atos approach.
Gemalto has hired Deutsche Bank and JPMorgan as its financial advisers, while law firm Allen & Overy is acting as its legal adviser. Rothschild & Cie and BNP Paribas advise Atos on the deal, sources said.
UBS analyst François-Xavier Bouvignies said that if Atos could achieve the same level of synergies from a takeover of Gemalto as it had from buying French computer equipment maker Bull in 2014, another 5 euros could be added to his valuation of 49 euros per share. ($1 = 0.8452 euros) (Additional reporting by Cyril Altmeyer, Patrick Vignal, Marc Angrand and Julien Ponthus in London; Editing by Richard Lough and Alexander Smith)