Germany's economics ministry has given the go-ahead for energy group RWE to sell its oil and gas unit DEA to a group of investors led by Russian tycoon Mikhail Fridman.

A clause in Germany's foreign trade law, would have allowed the ministry to stop the deal, announced earlier this year, if there were concrete signs it threatened "public safety and order," but the clause has never been invoked.

In a statement, the ministry said it did not expect the deal to negatively impact Germany's energy supply.

When it was first announced the €5.1 billion sale was criticized by senior German politicians in March, as relations between Russia and the West deteriorated over Ukraine.

The sale will hand Fridman, Russia's second-richest man, and other co-investors stakes in about 190 oil and gas licenses or concessions in Europe, the Middle East and North Africa.

Germany currently receives more than a third of its gas and oil from Russia. More than 6,000 German companies are active in the country and business associations and trade bodies have warned that an escalation in tensions over Ukraine would result in catastrophic losses for firms.

RWE also must obtain approval from other countries in which Dea has activities, including the UK, Norway, Denmark and Egypt. Germany's approval was regarded as the biggest challenge, however,

The European Commission signalled its approval in July.