William Hague has hailed implementation of an interim deal to freeze Iran's nuclear programme.
The British Foreign Secretary said the agreement coming into force on January 20 was an "important step" in settling the differences between the Middle East and international powers.
The deal, hammered out in November, will see Iran halt enrichment of uranium above 5% purity, "neutralise" its stockpile of 20% enriched uranium and provide daily access to inspectors.
Mr Hague said: "I welcome the fact that we have now reached E3+3 agreement with Iran on implementing the first step of the Joint Plan of Action agreed at Geneva on 24 November 2013.
"The entry into force of this agreement on January 20 is an important step towards peacefully resolving the Iranian nuclear issue, on which comprehensive negotiations will now start."
In his statement, President Barack Obama welcomed implementation of the agreement, but added: "We will continue to vigorously enforce the broader sanctions regime, and if Iran fails to meet its commitments we will move to increase our sanctions."