YESTERDAY was an extraordinary day of threats and counter-threats, even by the standards of the three-year row over Iran’s nuclear programme.

From the early morning, telephone calls rattled between London, Paris, Berlin, Vienna, Washington and Tehran, after Iran threatened to restart a controversial part of its work.

Jack Straw, the Foreign Secretary, said on the BBC’s lunchtime news that if Iran broke its promise to freeze this work it stood to lose favours offered only recently, such as support for its desire to join the World Trade Organisation. Michel Barnier, his French counterpart, spoke of “consequences”.

Most specific, Tony Blair said that if Iran went ahead he would back the referral of the case to the United Nations Security Council.

By the close of play yesterday in Tehran, it looked as though Iran was backing down, at least for a few days. There was no sign of a letter it had threatened to send the International Atomic Energy Agency (IAEA), the UN’s nuclear watchdog based in Vienna, announcing the resumption of the work.

At least, Sirus Naseri, Tehran’s chief delegate to the IAEA, had arrived in Vienna with the letter, but had not delivered it.

If so, the European negotiations have bought a bit more time. The best hope is that the latest act of this melodrama has been written for the theatre of Iran’s presidential elections on June 17, and that its leaders then return to a calmer script.

The row concerns uranium enrichment, a process which can make the fuel for nuclear reactors but also, with very little modification, the material for weapons.

Iran says it wants to master this technology to make its own fuel for the reactors it is building. Its critics say that its record of concealment of a 20-year nuclear programme, revealed by dissidents in 2002, suggests that it wants to make weapons.

Britain, France and Germany, the so-called “E3” group, have taken the lead in trying to persuade Iran to halt this work (in the face of US scepticism about such a “soft” approach).

Last year, under the Paris Agreement, Iran promised the E3 to halt all work on uranium enrichment. A resolution of the IAEA Board of Governors in November also insisted that it stop this work.

The current stand-off has been building for two weeks. Iran has been trying to persuade the E3 to let it keep an experimental line of research alive. The E3 has said no, on the grounds that this would enable Iran to master the technology.

Iran’s current threat is not to start enriching uranium, but to work on a preliminary step — producing a gas that is the feedstock for enrichment.

It had hoped that the E3 would accept its view that this was not really enrichment, but the E3 has not.

Yesterday the E3 said that it had sent a letter to Tehran, also signed by Javier Solana, the European Union’s foreign policy chief, saying that it was not going to budge on the interpretation of the work that must remain frozen.

It also spelled out the immediate consequences if Tehran does break the agreement.

According to a senior official in the E3 group:

If Iran actually resumes the work, then at a further meeting of the IAEA Board, the E3 and the US will press for a referral to the UN Security Council.

E3 officials think that this threat is much stronger than in the past because they believe that the IAEA Board, including Russia, would support it — and because they and the US are finally at one in agreeing to take this step.

However, in the letter, the E3 has also offered Tehran a ministerial-level meeting the week after next. There were signs last night that Tehran might be tempted to delay its decision at least until then.

Western officials believe that the crisis reflects tension within Iran over the imminent elections, but they differ on the reason why.

Some suggest that Ayatollah Ali Khamanei, the Supreme Leader, feels threatened by the aspirations of Hojatoleslam Ali Akbar Hashemi Rafsanjani, the former President, to stand again for that post. On this analysis, the Supreme Leader’s authority might be diminished by an elected leader of such established reputation — and he might feel himself boosted by an atmosphere of conflict with the West.

Others argue that Hojatoleslam Rafsanjani himself might find the conflict helpful before the elections.

The Western hope is that Iran will agree to maintain the freeze until the elections and that afterwards its stance will soften.

Times Recommends

• Air crash: ‘Everything is burnt. It is the closest thing to Hell’
• The true story of a passionate Korean 'spy'
• I shot Lennon in the back, Chapman tells parole board