The Ministry of Defence said last night that the problem had occurred after exceptional flooding in 1989 caused contaminated water in surface drainage pools on the Aldermaston weapons site to seep across to the 125-acre Blue Circle Cement site.
Blue Circle is concerned that it has only just been informed of the contamination although tests were carried out in August and October 1990.
Blue Circle said it was given assurances that the site was safe when the company moved into the four-storey office building on the landscaped site eight years ago.
The company employs 150 people on the site and its tenants employ a further 60 staff.
Ian McKenzie, the company's chief executive, said: 'Our over- riding concern has been for the safety of staff and others visiting Aldermaston Court.'
A Ministry of Defence spokesman said there was no health risk and that the level of contamination 'was very, very low - scarcely above background levels'.
He said arrangements were being made to remove the contaminated soil from the site.