LONDON - The British government on Monday prohibited UK companies' dealings with a major Iranian bank and state-owned shipping fleet in an attempt to put pressure on the Islamic Republic over its nuclear weapons program. The Treasury said that UK companies would stop doing business with Bank Mellat and Islamic Republic of Iran Shipping Lines (IRISL) adding it had enough evidence to show the institutions posed "a serious threat" to UK interests. Treasury Minister Sarah McCarthy-Fry said on Monday that the development or production of nuclear weapons in Iran poses a significant risk to the national interests of the UK in light of Iran's failure to meet its international obligations or to answer questions from the International Atomic Energy Agency. "The direction to cease business will therefore reduce the risk of the UK financial sector being used, unknowingly or otherwise, to facilitate Iran's proliferation-sensitive activities," a Treasury statement said. The Counter-Terrorism Act of 2008, which increased government and police powers in countering terrorism, allows the Treasury to direct the financial sector. The minister said that Bank Mellat had been "involved in transactions related to financing Iran's nuclear and ballistic missile program." The government has also called for a ban on IRISL stating it had "transported goods for both Iran's ballistic missile and nuclear programs." "Financial and credit institutions will no longer be able to enter into new transactions or business relationships with these entities, nor to continue with existing transactions or business relationships unless they are licensed by the Treasury," McCarthy-Fry announced. Bank Mellat is one of Iran's largest banks, nationalized in 1980 after the revolution in 1979, by the forced merger of 10 private banks. It is said to be Iran's second-largest bank with some $40 billion of assets. The US Treasury Department recently put the bank on its watchlist of organizations that may be trading in violation of a UN Security Council Resolution regarding money laundering.