Like riverboat gamblers casting loaded dice, Iran's leaders have played a double game of deceit, duplicity and Persian blind man's bluff in on-off talks with western countries since the existence of suspect nuclear facilities was first exposed. Now it seems the Iranian regime has been caught red-handed, and clean out of trumps, by the forced disclosure that it is building, if not already operating, a second, secret uranium processing plant.

The revelation will bring a triumphal roar of "told you so!" from Bush era neoconservatives in the US to hawkish rightwingers in Israel. The likes of former vice-president Dick Cheney and UN envoy John Bolton, and the current Israeli leader, Binyamin Netanyahu, have long insisted that Tehran's word could not be trusted.

Yet the argument about who was right and who was wrong about Iran is hardly important at this juncture. Today's disclosures have significant, real-time policy-shifting implications for those who must deal with the ever more believable claims that Iran is seeking nuclear weapons capability.

Britain has already changed tack. Foreign Office diplomats have been saying privately for months that bigger sticks would be needed this autumn, given Tehran's endless foot-dragging. What they knew, but could not say, about its clandestine activities is now in the public domain.

France and Germany can now be expected to stiffen their resolve too. That means the prospect of effective measures to curb private sector trading and financial transactions with Tehran as well as government and EU level sanctions. After President Ahmadinejad's latest bout of Holocaust denial, the German chancellor, Angela Merkel, was in any case in no mood to go easy.

Russia's new-found readiness to consider the "far tougher" sanctions demanded by Gordon Brown at the UN this week is doubtless linked to this confirmation of Iranian bad faith. But it also has an evident bilateral dimension in terms of Moscow's relations with the US.

All those who were writing off Barack Obama last week as a foreign policy lightweight may now reflect at leisure on how he has achieved two major objectives in almost as many days: Russia is back on side, for now at least, thanks to his decision to re-model European missile defence. And China is now isolated in the security council in opposing new sanctions on Iran – a position it always tries to avoid on any major issue, and which it may now find untenable.

Today's disclosure, and the concomitant conclusion that Iran's leaders are congenital double-dealers, will further spur the debate among regional neighbours, in particular Saudi Arabia, Turkey and Egypt, about acquiring nuclear capabilities of their own. Thus does the feared, fabled Middle East nuclear arms race inch closer.

The fact that Iran has been found out this time hardly makes much difference to the neighbours' strategic-defensive calculus; nor does the fact that Obama and Brown are pushing a western nuclear disarmament agenda with unusual vigour. The west's "nuclear umbrella" never covered the Arab states in any case. The question they will ask instead is what else is Tehran up to that they and the west do not know about? They will need reassurance. In the Middle East the balance of terror just shifted dangerously.

For its part Israel will be gratified that Iran, long its "existential" security issue, is now being treated with equal seriousness by western countries and Russia. Netanyahu and his rightwing cabinet will wait for the "crippling" action against Tehran anticipated by secretary of state Hillary Clinton. But for different reasons they will also remain sceptical, like China, about whether sanctions will work.

Netanyahu's deadline, imposed on Obama when he was looking the other way, of an end-of-year deadline for substantive progress in reining in Iran will remain in force, at least on the Israeli side. The threat thereafter of Israeli military action, against what appears to be an ever-expanding list of targets, remains in place, too.

Yet Obama's bid to open a dialogue with Iran – the so-called "unclenched fist" – may, conversely, be strengthened by these latest developments. Iran has been placed on the back foot. It has been caught lying, again. It cannot convincingly pretend, even to its central Asian friends and fellow travellers like Venezuela's Hugo Chávez, that it's a mere innocent victim of Washington's malice.

Nor can the regime easily tell its own much-abused, much-deceived people – and their moderate and reformist representatives in the Majlis – that it's all a foreign plot. This bombshell will weaken supreme leader Ali Khamenei, Ahmadinejad, chief negotiator Saeed Jalili, and the rest of Tehran's hardliner crew abroad and at home although, as usual, they will try to bluff their way through.

After months of frustration, Obama has forced an opening in Tehran's defences. It may not last for long; Tehran's sharpers and shysters may have more aces up their sleeves. But a fleeting opportunity to launch a genuine negotiation is there – if only he plays his cards right.