The former EU trade commissioner used a speech at the St Petersburg International Economic Forum to caution the answer to Russia's sluggish economic performance was not to turn its back on Europe.
As trade commissioner, Lord Mandelson negotiated a blueprint for an EU trade agreement with Russia, but this ended after president Vladimir Putin's attention turned to a customs union with former Soviet states.
Lord Mandelson said: "The Eurasian customs union, however important a political project, is not the answer. Less than 8% of Russian exports went to Belarus and Kazakhstan last year, compared to the 53% which went to the EU. These numbers speak for themselves."