The U.K. will press European Union (EU) leaders to consider blocking Russian access to the SWIFT banking transaction system under an expansion of sanctions over the conflict in Ukraine, a British government official said.

The Society for Worldwide Interbank Financial Telecommunication, known as SWIFT, is one of Russia's main connections to the international financial system. Prime Minister David Cameron's government plans to put the topic on the agenda for a meeting of EU leaders in Brussels tomorrow, according to the official, who asked not to be named because the discussions are private.

"Blocking Russia from the SWIFT system would be a very serious escalation in sanctions against Russia and would most certainly result in equally tough retaliatory actions by Russia," said Chris Weafer, a senior partner at Moscow-based consulting firm Macro Advisory. "An exclusion from SWIFT would not block major trade deals but would cause problems in cross-border banking, and that would disrupt trade flows."