It would have been a "grave error" for western countries to walk away from the negotiating table with Iran, William Hague has told MPs.
Israel's prime minister Binyamin Netanyahu has dismissed the deal agreed at 04:00 this morning as a "historic mistake".
But Britain's foreign secretary insisted the agreement was a "significant step towards enhancing the security of the Middle East and preventing nuclear proliferation worldwide".
The interim deal will ease sanctions on Iranian petrochemicals, gold and precious metals, help Iranian automobile and car companies and establish a financial channel for humanitarian aid and legitimate trade.
In return Iran is expected to halt the enriching of uranium beyond levels consistent with domestic energy needs and in some areas is even set to roll back its steady progress towards developing a nuclear weapon.
The EU and US' sanctions on oil and gas will remain in place and will not be lifted until a comprehensive agreement is reached next year.
"The fact we have achieved for the first time in nearly a decade an agreement that halts and rolls back Iran's nuclear programme should give us heart this work can be done and that a comprehensive agreement can be obtained," Hague told MPs.
"On an issue of such complexity, and given the fact that to make any diplomatic agreement worthwhile on both sides it has to involve compromising, such an agreement is bound to have its critics and opponents.
"But we are right to test to the full Iran's readiness to act in good faith, to work with the rest of the international community and to enter into agreements."
He warned Iran against a failure to comply with the terms of the deal but insisted the west was right to pursue the negotiations.
"If we did not take the opportunity to attempt such an agreement, then we ourselves would be guilty of a grave error," he added.
Shadow foreign secretary Douglas Alexander said the interim deal was "a necessary and important first step".
But his bipartisan approach was undermined by a complaint that Hague should have offered more prominent praise to the EU's foreign policy chief, Labour peer Cathy Ashton.
Those on the opposition benches took "particular pride" in her contribution, which Alexander said was "indispensable in ensuring agreement was reached".