The EU's top court has ruled that the EU should unfreeze the assets of seven Iranian banks and other businesses hit by sanctions.
The European Court of Justice (ECJ) said there was insufficient evidence that the businesses concerned were involved in nuclear proliferation.
Post Bank Iran, Iran Insurance Company and Export Development Bank of Iran are among those listed by the ECJ.
EU and US sanctions are aimed at curbing Iran's nuclear programme.
Western powers suspect Iran of trying to develop nuclear weapons, so they have frozen the assets of various Iranian businesses thought to be linked to that programme.
Iran insists that its nuclear technology is solely intended for civilian uses.
Correspondents say the ECJ ruling is a blow to Western efforts to exert pressure on Iran over its nuclear programme.
However, EU foreign policy chief Baroness Ashton made upbeat remarks on Friday about resuming nuclear talks with Iran.
She said she had spoken to Iran's Foreign Minister Javad Zarif, a Western-educated diplomat who will head the Iranian delegation, after years when security hardliners had set Tehran's nuclear agenda.
She will meet Mr Zarif in New York this month, on the sidelines of the UN General Assembly, to discuss a new date for the nuclear talks, she said.
"We've got some good proposals on the table. We really want to move now quickly to resolve this," Lady Ashton added.
Iran's new president, Hassan Rouhani, is seen as more moderate than his predecessor Mahmoud Ahmadinejad, and he has signalled a desire to break the deadlock in the nuclear talks.
Biggest bank still hit
The EU governments, jointly called the Council, have two months within which to appeal against the ECJ decision.
So for the time being the asset freeze will remain in place, the court said in a statement.
The other Iranian businesses that come under Friday's ECJ ruling are Good Luck Shipping, Persia International Bank, Iranian Offshore Engineering & Construction Co, Bank Refah Kargaran and one individual - Naser Bateni, manager of the Hanseatic Trade Trust & Shipping (HTTS), based in Hamburg.
The court found, however, that the asset freezes imposed on Bank Melli Iran and Europaeisch-Iranische Handelsbank were justified.
Bank Melli Iran is Iran's biggest bank. The court said the bank had "ensured that scholarships were paid on behalf of the Atomic Energy Organisation of Iran (AEOI) after restrictive measures had been adopted against AEOI by the United Nations Security Council" - a fact which "constitutes support for nuclear proliferation".
The bank's London-based subsidiary Melli Bank Plc has assets worth more than $3.5bn (£2.2bn), according to the company website.
Of those whose assets were unfrozen by the court ruling, Post Bank Iran is one of the most important.
Owned by the state, it specialises in rural banking and boasts 10,000 small rural branches.
"As regards accessibility in remote areas, it is the most widely accessible bank in Iran," the bank claims.
Persia International Bank is registered as a UK bank and has only two shareholders - the big Iranian banks Mellat and Tejarat.
State-owned Iran Insurance has more than 200 branches all over Iran, as well as overseas branches, and boasts "the largest sales network within the Iranian insurance industry".
Bank Refah Kargaran is a commercial bank with around 1,100 branches in Iran.