The company said that it had analysed the takeover offer published late last week and that, based on a fairness opinion prepared by Bank J. Safra Sarasin, the Nationale Suisse board "judges the offer price to be reasonable and therefore unanimously proposes the acceptance of Helvetia's offer by the shareholders".
The CHF1.8bn deal was announced on July 7. On completion the enlarged company will be the third-largest insurer in Switzerland in terms of premiums written.
The combined operation will have annual premiums of about CHF9bn, while Helvetia has predicted annual cost savings of between CHF100m and CHF120m.
Nationale Suisse said that the foreign operations of the two companies "complement one another well" and would have a combined premium volume of around CHF3bn "in the most important European retail markets".
Helvetia is an all-lines insurer with annual premiums of about CHF5bn. The Swiss market will remain the main pillar of the merged group, with a balance of life and non-life business.
Under the agreement, Nationale Suisse Specialty Lines operation will continue as an independent business, to be strengthened through the integration of selected business lines from Helvetia. David Ribeaud will be head of the new Specialty Lines business area. He will become a member of the Helvetia Group's management.
The Specialty Lines unit will bring together marine/transport insurance, engineering, art and the active reinsurance divisions of Helvetia and Nationale Suisse. That includes the current Helvetia France. The new enlarged operation will have premium volume of about CHF1bn a year.
The life business and international specialty lines will be managed from Basel. A regional non-life hub will also be created in Basel.
Nationale Suisse's current management team "will mostly be integrated within the new organisation".
Andreas von Planta, Balz Hösly, Peter A. Kaemmerer and Gabriela Maria Payer from Nationale Suisse will become members of the Board of Directors of Helvetia. Hans Künzle will hold the post of second Vice-Chairman of the Board of Directors.