Bonuses for executives at Société Générale were boosted by the successful investments of Jérôme Kerviel, the rogue trader, his lawyer said yesterday.

Speaking as part of a campaign to have the trader released from jail, Guillaume Selnet reiterated Mr Kerviel's assertion that he had worked alone on the unauthorised trades that cost the French bank €4.9billion (£3.6billion).

Mr Selnet added: “Although he was the youngest trader, he was by far the most profitable of the team. He was only entitled because of his age to a nominal bonus. What we can say so far is that apparently the rest of the team, including the senior members, was served bonuses based on Jerome's trades.”

SocGen declined to comment.

Mr Kerviel, 31, has been held in La Santé prison, Paris, for a week to prevent any contact with possible accomplices while an investigation into his case continues. His lawyers have lodged an appeal for his release, arguing that nobody else was involved in the trades, but that SocGen had been aware of his activities.

The trader worked on plain vanilla futures, but secretly bet €50billion on the movements of European indices. SocGen spent three days unwinding the positions at a huge loss.

Since the scandal emerged last month, the French bank has been seen as a takeover target, but sources close to SocGen's banking advisers - Merrill Lynch, Morgan Stanley and JPMorgan - said yesterday that the chances of BNP Paribas, its main rival, bidding for SocGen appeared to be receding.

One source said that if BNP had been planning to pounce, it would have been more logical to do so before SocGen moved to strengthen its balance sheet this week by launching a €5.5billion rights issue. Now, with the share sale under way, BNP must wait until after February 29 before making any move.

Others said that the complexity of the integration of the two banks could be enough to deter BNP. Unlike American banks, which outsource much of their technology needs, French banks have home-grown, proprietary systems that are incompatible. The bitter rivalry between the two banks and their unions would also make a deal difficult to achieve, both culturally and because a merger of the two would involve thousands of job cuts.

“Everyone at SocGen hates BNP, so the integration would be a bloodbath,” one source said.

Lawyers are convinced that any merger or break-up would be referred to the French anti-trust regulator and most likely to competition watchdogs in Brussels.

Dominique Brault, a competition partner with Herbert Smith, a law firm in Paris, said: “There was a debate about whether French banks were subject to French merger control, but that concluded in 2003 when a special law provided that banks would be subject to the rules just like any other company.”

One source close to BNP said: “They are still thinking about it but they haven't made a decision yet. Nobody wants to be the first mover, because the first mover never wins and BNP's not on a suicide mission.”

Crédit Agricole has hired Lazard to advise it on any bid, but it is understood that the retail banking giant - which has about 30 per cent of the French market - would show its hand only after BNP made an offer. “It wouldn't tolerate a tie-up between them but it won't move proactively either,” a source said.

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