The threat of a nuclear catastrophe is higher now than at any time since the most tense days of the Cold War, a leading international think tank has heard. Plummeting US-Russia relations, threats from terrorists and rogue states and, significantly, the behaviour of US President Donald Trump are key factors.

Mr Trump’s shadow loomed over the 10th-anniversary conference of the Luxembourg Forum with speakers expressing grave worry over Mr Trump’s avowed aim of scrapping the landmark nuclear deal with Iran and his repeated public threats to annihilate North Korea.

The failure of Washington and Moscow to negotiate on their nuclear arsenals amid a series of geopolitical confrontations ranging from Ukraine to Syria has created uncertainty and instability, the meeting in Paris was told. But it was impending action by Mr Trump which was the source of deepest and most immediate concern to delegates.

Mr Trump is expected to announce this week that he will not certify the agreement with Tehran, starting a process which may lead gains of years of painstaking diplomacy unravelling. Statesmen at the summit warned that this would also have the highly damaging consequence for attempts to halt North Korea’s nuclear programme with Pyongyang claiming that an international deal can be sabotaged by the US in the future.

Addressing the Forum, Tony Blair acknowledged that aspects of the agreement, the Joint Comprehensive Plan of Action ( JCPOA) with Iran, were questioned when it was signed. However, he continued: “But now it has been done, it has a process of verification – it means that for now at least the nuclear programme of Iran can be stalled, and the sensible thing, in my opinion, is to preserve it.” The Middle East, he added, was going through a period of great strife and “abandoning the JCPOA would add another dimension of immediate risk which at present we don’t need”.

The former British Prime Minister described the North Korean regime as “abhorrent”. But diplomacy, with the help of China, and not sabre-rattling was the right avenue to stop Kim Jong-un acquiring a nuclear arsenal. “We can threaten military action… But unless there are elements of which I am unaware in either the weakness of the North Korean defences or the strength of US capabilities, it is hard to think of a pre-emptive strike which would not result in catastrophic consequences”, said Mr Blair.

William Perry, the highly respected former US Defence Secretary who had served under Presidents Carter, Reagan and Clinton, said he was “appalled” by the rhetoric used by Mr Trump over the North Korean crisis: “Leaving aside the matter of political decorum I think this kind of language stimulates and creates the kind of condition which may mean that the US and North Korea may blunder into a war. And the results of such a war would be catastrophic.”

Mr Perry dismissed Mr Trump’s claims about the supposed failure of the Iran nuclear deal. “I do not agree with his views about the agreement. We can see that the agreement has been successful in significantly curtailing the Iranian nuclear programme, to the benefit of our security,” he stated.

“President Trump seems to think that the agreement could be scrapped and then renegotiated. But another agreement is not something which would be available. Scrapping the deal would have an effect everywhere including Europe, among our allies, who know that it is working.”

Hans Blix, the former head of the International Atomic Energy Agency, said: “President Trump recently spoke of his regard for the UN. The UN has said that Iran is complying with the agreement. Mr Trump now wants to walk away and unilaterally tear up this agreement, saying it is not working. But the fact is that the Iran deal is far-reaching and in place... reached multilaterally and other countries believe it is working and they will try to preserve this agreement.”

Viatcheslav Moshe Kantor, the President of the Luxembourg Forum, made clear that “undoing the Iran nuclear deal would be unforgivable”. He also warned that the possibility of a deliberate provocation by North Korea was “very real' and could trigger a ”global chain of nuclear strikes,“ with America needing to tread carefully over both issues.