MILAN/SIENA (Reuters) – Monte dei Paschi di Siena said the closure of a derivatives contract with Nomura by end-July, as requested by the European Central Bank, could trigger a pre-tax loss of around 1 billion euros ($1 billion), according to a letter from the bank to prosecutors. Closing Alexandria would stop the bank pressing ahead with any damage-seeking proceedings and would weaken its case in a criminal probe under way in Milan into the trade, it said.