The development follows more than two weeks of wrangling between Iranian diplomats and Western negotiators over preparations for the closely-watched round of nuclear talks which had cast some doubts over what can be achieved in Moscow.

A tense exchange of letters between EU diplomats, who deal with Iran on behalf of the six powers, and Iranian officials had earlier appeared to suggest Tehran may be backtracking on its expressed willingness to discuss their most pressing concern – high-grade uranium enrichment even in broad terms.

But on Monday, Iran’s chief nuclear negotiator Saeed Jalili agreed to focus on the six powers’ demands at the Moscow meeting, during a one-hour phone conversation with EU foreign policy chief Catherine Ashton.