The Presidents of Kazakhstan, Belarus and Russia The union's partners cannot agree on the issue of export duties
A customs union between Russia, Belarus and Kazakhstan will effectively come into force on Tuesday after Minsk ratified a key customs code.
The three-state union should have been launched on 1 July, but Belarus had refused to sign the documents.
Minsk wanted Russia to drop its export duty on oil and oil products for its union partners.
Belarus eventually ratified the code last week despite the fact that the duty dispute remains unresolved.
Russian President Dmitry Medvedev confirmed the move at a summit in Kazakhstan on Monday.
"A statement was signed on the enforcement of the customs code.
"In relations between the three countries it will be applied as of 6 July this year, in relations between the Russian Federation and Kazakhstan as of 1 July 2010."
Speaking of export duties, Russian First Deputy Prime Minister Igor Shuvalov said: "The Belarusian side is constantly raising the issue that establishing a customs union and a joint customs territory certainly leads to abolishing export duties in trade."
But he insisted that export duties between the union's partners would only be abolished after the Single Economic Space was established in 2012.
The aim of the customs union is to create a single market of some 170 million people, boosting trade and investment between its members.
At Monday's summit, Kyrgyzstan and Tajikistan said they were studying the possibility of joining the union.