Referring to the murder as a "grave and reckless crime", the EU stressed that "urgent and constructive cooperation by the Russian Federation" is crucial, in a statement issued on Wednesday (18 July).

The presidency declaration is being viewed as an important signal of support for the UK, in a row that Moscow has tried to keep exclusively between itself and London.

It also comes despite Portugal earlier indicating that it does not want to increase tensions with Russia during its term at the EU helm, running until the end of the year.

Earlier this week, Alexander Grushko, the Russian Deputy Foreign Minister, was reported in British media as saying: "We hope that common sense will prevail and its members will not yield to more attempts to turn relations between Russia and the EU into an instrument for one-sided political goals that have nothing to do with the real interests of co-operation."

But Portugal insisted the matter "raises important questions of common interest to EU Member States" and called for its "satisfactory" solution. The issue is expected to be raised next Monday (23 July) when EU foreign ministers gather in Brussels.

The diplomatic row has so far resulted in Britain expelling four Russian embassy staff in reaction to Moscow's reluctance to extradite Mr Lugovoi.

Russia argues that it is against the country's constitution to hand over its citizens for trials in other countries. The UK counters that an international accord signed by Moscow, paves the way for the hand-over.

The case is sensitive due to a statement by Mr Litvinenko, unveiled a day after his death, accusing Russian President Vladimir Putin of involvement in his poisoning. The Kremlin strongly rejects any involvement in the murder.