The moves follow the disclosure of potential losses of pounds 900m from unauthorised currency trading at the Japanese company over a four-year period.
The potential losses were run up on an estimated dollars 6.4bn speculation on dollar and yen exchange rates by the company's treasury staff and were in breach of in-house regulations that restrict them to hedging against currency fluctuations.
Such hedging is regarded as normal for many big companies trading in international markets. But Showa's dealings were far bigger than usual and involved taking massive positions on forward foreign exchange contracts.
Showa, which is listed on the Tokyo stock market, is under pressure from Shell to dismiss those responsible. Shell nominates half of Showa's directors.
There is growing speculation that Shell, which is carrying out a high-level investigation into the foreign exchange dealings, will announce some staff departures when it releases its 1992 annual results on Thursday.
The company said it viewed the matter 'with concern' and supported the actions of Showa 'to ensure that such a situation will not occur again'.
Shell has estimated that the currency contracts will result in a net charge of pounds 131m in its 1992 results. The company is expected to report net profits of about pounds 3bn.
However, the disclosures shocked City analysts. Jeremy Hudson at Lehman Brothers said: 'Shell is regarded as a well-ordered company and it is surprising something like this has happened.' Shell's shares fell 6p to 575p.Reuse content