Britain is attracting around a billion pounds ($1.5 billion) of capital inflows a month not recorded by official statistics, up to 40 percent probably from Russia, according to a Deutsche Bank study on Tuesday.

Among the reasons behind "net errors and omissions" (NEO) from official data are misreporting by financial institutions, sophisticated tax avoidance and accounting methods, and Britain's perceived "safe-haven" status, the report said. The total "NEO" inflow into the UK since the mid-1990s is around 133 billion pounds. Some 93 billion pounds of that has come since 2006, with the pace accelerating since 2010 to around 1 billion a month, according to the report.

Its findings are part of a broader study of NEOs across major economies. This "dark matter" represents financial flows that don't show up in official balance of payments data but could have big implications for foreign exchange rates. "For the first time, we confirm through balance of payments data the popular belief that Russian money has flooded into the UK in recent years," the report said.

"Hidden inflows ... raise major questions over the UK's role as a refuge for international capital flight and the need for better data and more rigorous oversight on property transactions," it said.

The slump in oil prices and Western sanctions over Moscow's perceived role in the conflict in Ukraine have pushed an already-slowing Russian economy into an expected deep recession this year, triggering a rush of money out of the country.