The eight men arrested in the Financial Services Authority's largest investigation concerning insider dealing will face further questioning after being released without charge yesterday.

The eight, who include a contractor at Cazenove and a junior UBS employee, have been bailed to reappear at a police station, the FSA said.

Releasing suspects without charge is standard practice in such cases because of the complex nature of investigations. The FSA declined to give further details about the group of suspects, who are aged between 27 and 48. City sources indicated that at least one worked for a printing business involved in the production of sensitive bank information such as takeover documents.

The arrests on Tuesday indicated that the regulator had entered a more aggressive phase in its battle against insider dealing. The swoop on homes in London and the South East involved 40 FSA enforcement staff backed by officers from the City of London Police. The raids came after a recent flurry of activity from the regulator, which has faced persistent criticism for being too reluctant to use its powers to bring criminal prosecutions.

The FSA already has three criminal cases under way including one in which it has charged Malcolm Calvert, a former Cazenove partner, with trading on illegal information he received after retiring from the stockbroker.

Jason Mansell, a barrister at 7 Bedford Row, said that the recent spurt of criminal investigations showed the FSA was prepared to use all the tools in its toolkit to fight financial crime.

The maximum penalty for insider trading is seven years. The FSA has never secured a conviction since it took responsibility for policing insider trading in 2001. One reason it has traditionally shied away from prosecutions is that offences are difficult to prove. To secure a conviction it must convince a jury that a defendant knew that he was using inside information and that the only reason he traded was on the basis of that information.

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