Aviva has turned down an offer of £5 billion ($8.7 billion) from RSA for its general insurance business in the UK, Ireland and Canada.

RSA’s proposal would create the biggest non-life insurer in the UK and end Aviva’s aim to sell every type of insurance product. The London-based company is the only major UK insurer to sell life and super products alongside motor and home insurance.

Aviva, the second-biggest insurer behind Prudential in the UK, has lost 52% of its value since 2007 and the financial crisis dawned. RSA fell 12% in the same period.

RSA says its bid has been considered for some time and makes strong strategic sense, with significant cost synergies estimated at £300 million ($519 million) a year.

It says the resulting businesses would be in the interests of both sets of shareholders.

“RSA considers that its proposal represents fair value for the target businesses and would be in the interests of both sets of shareholders. [We] remain open to discussions with Aviva,” RSA said in a statement.

Aviva has a market value of £11 billion ($19 billion) but now needs to prove to shareholders that the decision to reject the RSA bid is the right move.

Commentators say RSA will probably have to raise its offer 20% to £6 billion ($10 billion) to reopen negotiations with Aviva.