UBS said it had concluded in February that buying its stricken rival Credit Suisse was not desirable, but that it should prepare in case the lender encountered "serious financial difficulties".
In a filing to the US Securities and Exchange Commission, UBS said it had been considering the potential impact of a Credit Suisse deal since December.
The UBS disclosure, dated April 26, provides the clearest insight yet into UBS's thinking and shows it had been looking at its struggling competitor months before its hastily arranged rescue by Swiss authorities.
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In March, UBS agreed to take over Credit Suisse for 3 billion Swiss francs ($3.37 billion) and said it would assume up to 5 billion ($5.61 billion) in losses, as part of a rescue that is backed by as much as 250 billion Swiss francs of state support. The Swiss authorities and UBS Group AG have been racing to close the takeover as soon as possible in an effort to retain the lender's clients and employees, Reuters has reported.
UBS said in the filing that the merger still required approval from regulators in the European Union, India, Japan, Mexico and South Korea.
Last month, UBS secured temporary approval from European Union antitrust regulators, while the U.S. Federal Reserve approved the UBS Group's acquisition of Credit Suisse's U.S. subsidiaries.