FOREIGN Secretary William Hague has hit back at critics of the international deal with Iran on its nuclear ambitions and told MPs it would have been a "grave error" not to attempt a diplomatic deal.

Critics of the agreement, finally brokered over the weekend after marathon talks in Geneva, would inevitably point to the easing of about £4.3 billion of sanctions on Iran, Mr Hague said.

But in a statement to MPs, he said reaching a deal for the first time in a decade was a milestone on the path towards a full deal to halt Iran's nuclear weapon ambitions for goods.

Mr Hague said: "The fact we have achieved for 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 attained.

"It is true that, if we did not have this agreement, the pressure of sanctions on Iran would not be alleviated at all. But it is also true there would be no restraint on advances to their programme, no check on their enrichment activity and stockpiles, no block on their addition of centrifuges, no barrier to prevent them bringing into operation their heavy water reactor at Arak and no limitation on the many actions that could take them closer to a nuclear weapons capability."

Shadow foreign secretary ­Douglas Alexander said the deal was a "necessary and important first step".

He added: "This is not a perfect deal, nor is guaranteed to lead to a comprehensive resolution.

"But, based on your statement to the House today, it appears to address a number of central concerns."

Mr Hague said: "The bringing together of this agreement with all five permanent members of the security council united behind it in itself sends a powerful signal.

"So while it is only a beginning there is no doubt this is an important, necessary and completely justified step which through its restrictions on Iran's nuclear programme gives us the time to negotiate a comprehensive settlement."

Mr Hague paid tribute to ­Baroness Ashton, the European Union's High Representative on foreign affairs, and to Jack Straw, the former Labour foreign secretary who launched talks with Iran a decade ago.

He said Foreign Office staff played an "indispensable role" in the talks, which Mr Hague joined on Saturday alongside US Secretary of State John Kerry.

Mr Hague said Iran had made "extensive commitments" on its nuclear facilities and said the joint plan of action agreed had an end goal of "ensuring its nuclear programme is for exclusively peaceful purposes".

The six-month agreement can be renewed by mutual consent and sets out actions to be taken by both sides, Mr Hague said.

Mr Hague said sanctions would be suspended on oil-related transport and insurance costs, and on Iranian imports of gold and precious metals.

He added: "But core sanctions on Iranian oil and gas will remain in place. It is intended the EU will increase by an agreed amount the authorisation thresholds for financial transactions for humanitarian and non-sanctioned trade with Iran."

Mr Hague said no new nuclear related sanctions would be adopted by the UN, EU or US during the six-month agreement.