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LONDON, Oct 12 (Reuters) - Britain ordered financial firms on Monday to halt all business relations with two Iranian companies because of concerns that they were involved in helping to develop nuclear weapons.

The order applies to Iran's Bank Mellat and Islamic Republic of Iran Shipping Lines.

A statement issued by the Treasury said it welcomed recent talks between six world powers and Iran on the nuclear issue but that action was needed now on the two businesses.

"... Iran continues to pursue its proliferation sensitive nuclear and ballistic missile activities in defiance of five UN Security Council Resolutions," the Treasury said.

"On the particular entities in question, vessels of the Islamic Republic of Iran Shipping Lines (IRISL) have transported goods for both Iran's ballistic missile and nuclear programmes."

"Similarly, Bank Mellat has provided banking services to a UN listed organisation connected to Iran's proliferation sensitive activities, and been involved in transactions related to financing Iran's nuclear and ballistic missile programme," it added.

The order was not part of existing U.N. sanction against Iran but was taken under the Counter Terrorism Act 2008. Its publication coincided with the return of lawmakers to parliament after their long summer break.

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