Rigorous new sanctions against Iran's banking, shipping and industrial sectors took effect on Saturday, as part of the European Union's effort to force Tehran to scale back its nuclear program.
The sanctions, agreed in October, entered EU law with their publication in the European Union's Official Journal on Saturday.
The toughest EU measures yet, they include bans on financial transactions, sales to Iran of shipping equipment and steel, and imports of Iranian natural gas, adding to earlier bans, including on the OPEC producer's oil.
They reflect heightened concern over Iran's nuclear goals and Israeli threats to attack Iranian atomic installations if diplomacy and other measures fail to deliver a solution.
Diplomats say they hope talks with Iran can resume in January, but are waiting for an answer from Tehran, which maintains its nuclear program is for purely peaceful purposes.
In a statement, Britain's foreign office said there was a clear need "for an urgent solution".
"Iran's leaders know that sanctions are having a significant impact," Britain's Minister for the Middle East and North Africa Alistair Burt said.
"They should be in no doubt that the international community will keep up the pressure until they are ready to negotiate in good faith and take the concrete steps needed to convince the international community that they are not building a nuclear weapon."
The new sanctions mark a significant change of policy for the 27-member bloc, which previously sought mainly to target specific people and companies with economic restrictions.
It has lagged the United States in imposing blanket industry bans because it is anxious to avoid penalising ordinary Iranian citizens, while punishing the Tehran government.
Sanctions have increasingly inflicted severe pain on the Iranian economy, although the country has years of experience of circumventing them by using front companies and tortuous shipping routes.
The new European measures make clear natural gas shipments are prohibited in any form and swapping, as opposed to simply buying, cargoes is also outlawed.
While imposing a general ban on financial transactions, they make exceptions for those involving humanitarian aid, food and medicine purchases and provisions for legitimate trade.
In a statement, the European Commission said the new law brought the number of entities subject to sanctions to 490 and the total number of persons to 105.
The latest companies added to the banned list include energy and steel distribution firms and financial companies.
The latest individual to be added is Babak Zanjani, owner of the Sorinet Group, based in the United Arab Emirates. He is referred to as "a key facilitator for Iranian oil deals and transferring oil-related money".
Iran says its nuclear project has only peaceful energy purposes and has refused in three rounds of talks since April to scale back its uranium enrichment activity unless major economic sanctions are rescinded.