Philip Hammond is MP for Runnymede and Weybridge, and Secretary of State for Foreign and Commonwealth Affairs.

This week, I visited Iran to re-open our Embassy, the first time a British Foreign Secretary has set foot on Iranian soil in 12 years. After an absence of almost four years, the Union Flag is once again flying proudly in Tehran. The visit, accompanied by a strong delegation of leading British business people, sent a clear signal to the Iranian Government and the Iranian people of Britain’s re-engagement with Iran.

The decision to re-open our Embassy has not been an easy one. It is less than four years since militant protesters scaled the walls, ransacked some of our offices, burned the Union Flag – and nine of our local staff were arrested, despite their obvious innocence. For all the Foreign Office staff present at the time, it was a harrowing experience and many of the buildings in the Embassy compound still bear the scars of those dark days in 2011.

But much has changed since then, not least the demise of the hard-line Ahmadinejad regime. Two years ago, President Rouhani (an alumnus of Glasgow Caledonian University) was elected with a mandate for change in Iran. His Government has, so far, pursued a more moderate path than that of his predecessor. And last month, following more than a decade of painstaking and protracted negotiations, the UK – along with China, France, Germany, Russia and the United States – signed a potentially historic agreement with Iran to limit its nuclear programme to purely peaceful ends.

It is now 12 years since it was revealed that Iran was concealing nuclear activities, in violation of its international obligations – activities that many suspected were intended to lead to the development of a nuclear weapon. The united response of the international community – through multiple United Nations Security Council resolutions and, over time, the application of a strict and robust sanctions regime – succeeded in bringing Iran to the negotiating table.

In Geneva, Lausanne and Vienna, I and my counterparts spent many long hours and days negotiating with Iranian Foreign Minister Zarif on complex nuclear issues. Now, having reached a comprehensive agreement, our focus is on ensuring that the terms of the agreement are fully implemented, closing off all possible Iranian routes to a nuclear bomb for many years to come.

Iran is a large, well-educated and resource-rich economy. Its population is young and eager for re-engagement with the world. The media image of a country dominated by ageing theocrats is a far cry from the reality of modern Iranian demography. So our sincere hope is that the nuclear agreement will have much wider, positive consequences on Iranian society. As long as Iran fulfils its commitments under the nuclear agreement, global sanctions relating to its nuclear programme will be lifted allowing Iran to re-engage economically with the world. That re-engagement will bring great benefits to the Iranian people, and it will offer significant business and trading opportunities.

I am determined that, having born some of the impact of sanctions, British business should be well-placed to benefit when sanctions are lifted. That is why I took with me to Tehran senior UK trade experts and a business delegation with representatives from the engineering, architecture, banking and oil and gas sectors, among others, to start to make the business-to-government connections that could be vital in securing future trading opportunities. Some say that Britain has lagged behind in exploiting the commercial opportunities. Nothing could be further from the truth. There are, as yet, no deals to be done as sanctions are likely to remain in place into next year. But we have already helped business establish links with the Oil and Trade ministries, and with the Central Bank of Iran. And Francis Maude, the Trade Minister, will lead a further trade delegation in the coming weeks.

Our renewed Embassy presence will be an important resource on which our businesses can call: helping them to navigate effectively the Iranian system, reach the parts of the Iranian bureaucracy that are normally impenetrable to outsiders, and advocate for UK Plc. Over time, British businesses will profit, and the Iranian people will finally feel the benefits of the thaw in Iran’s relations with the world.

There are, of course, those who say that Iran’s continued interference in the affairs of its neighbours and the wider region should preclude that diplomatic thaw, and that now is not the time to renew relations with Iran. I do not agree. I have no illusions about the challenges: Iran’s interference in regional affairs; the continued violent rhetoric towards Israel from some of the leadership; its support for terrorist groups, as well as ongoing support for Assad, all remain sources of deep concern. I am in no doubt that in many areas, Iran’s outlook and priorities are very different from our own. But refusing to capitalise on the momentum created by the nuclear deal; standing back while our competitors increase their trade links, and refusing to re-engage with Iran would be a perverse response to the progress we have made in recent months. We will continue to disagree on many issues, but at least we can begin a dialogue and seek to find some areas of common ground.

By re-opening our Embassy now, we can use our diplomacy to encourage Iran to play a more positive role in the region, seeking to draw it into the international system as well as boosting British trade and investment.

The momentum provided by the nuclear agreement provides the opportunity for Iran to build a different kind of security and diplomatic relationship with its neighbours in the Middle East, and with the West. If mutual trust and confidence can, gradually, be built, there is an opportunity for Iran to re-align its approach in support of the international community’s efforts where our interests coincide, for example in tackling ISIL. And, in due course, we may be able to work together for compromise solutions to achieve resolution of the crises in Yemen and Syria. Of course, this process will take time. And, in the meantime, we are not starry-eyed about the Tehran regime.

But the nuclear deal has shown that patient diplomacy can deliver results, even on the most challenging of dossiers. With an Embassy in Tehran again, British diplomats will now engage with Iran fully: to seek to co-opt it in the struggle against ISIL; to speak candidly about human rights; and to build a trade and investment relationship that brings benefits to both our countries and our citizens.

Iran has a proud history and culture, and a highly educated population. Past problems were not with the Iranian people, but with the hard-line regime that claimed to represent them. Now the situation is changing, and we have an opportunity to re-engage. Now is the time for Britain to be back in Tehran, to play our part in what could be, if Iran takes the right path, a vibrant and successful future.