Iran on Wednesday welcomed a British court ruling against the extradition of former Iranian ambassador Nosratollah Tajik, wanted by Washington for allegedly conspiring to smuggle arms to the Islamic republic.

"The Islamic Republic of Iran welcomes the court ruling, even though it was delayed for six years," foreign ministry spokesman Ramin Mehmanparast said, quoted by state broadcaster IRIB.

Tajik, Iran's former ambassador to Jordan, was arrested in 2006 in Britain over claims he conspired to export US night-vision equipment to Iran without a licence.

At the High Court in London on Tuesday, judge Alan Moses said that Tajik would avoid extradition because Washington had failed to justify delays in the long-running case.

Mehmanparast called for Tajik to be released and allowed to return to Iran "as soon as possible."

British and US authorities have 14 days to lodge an appeal in Britain's highest court, the Supreme Court.

The extradition process has also been held up over fears that sending Tajik to the US could further damage British-Iranian relations.

The two countries have had a tense relationship since Iran's 1979 Islamic revolution, and Britain's embassy was stormed in November last year by protesters angry at British sanctions against Iran over its disputed nuclear programme.

Tajik is obliged to wear an electronic tag and submit to a night-time curfew at his west London flat.