How is a UK debt to Iran connected to Nazanin Zaghari-Ratcliffe?

Nazanin Zaghari-Ratcliffe - a British-Iranian woman who's been held for nearly six years in Iran - has been released and is on her way to the UK, according to her local MP.

Iranian press reports claim that her release is linked to the payment by the UK of a decades-old debt over an arms deal that was never fulfilled.

The money was paid to International Military Services (IMS), a private company, then a subsidiary of the Ministry of Defence.

Only 185 of the tanks were delivered to the Iranian authorities before the Shah was deposed in the Islamic Revolution of 1979.

The Iranian government has been trying to regain the money for the undelivered tanks ever since.

In 1990, Iran took IMS to the International Chamber of Commerce (ICC), a global business organisation which rules on trade disputes between countries.

In 2001, a hearing found in favour of Iran. IMS appealed, although in 2002 it agreed to pay £328.5m to the ICC, in case their appeal was unsuccessful.

The appeal hearings concluded in 2009 and the decision was upheld in favour of Iran.

However, the European Union had placed sanctions on Iran in June 2008, and IMS's payment was not released to the Iranians.

The UK government has admitted it owes Iran for the undelivered tanks, but said it could not make the payment while Iran was still subject to economic sanctions.

In addition, there was the complicating factor of how much the UK owed Iran.

The £328.5m security payment made by IMS in 2002, has attracted a considerable amount of interest since then. The UK disputed how much of this should be paid to Iran.

The British-Iranian was detained in April 2016, and has spent nearly six years in detention for allegedly plotting against the Iranian government. She protested her innocence throughout this time.

Ms Zaghari-Ratcliffe said she had been told by her captors that her imprisonment was connected to the fact that the UK had not paid its debt.

The former foreign secretary, Jeremy Hunt, also said that Iran was linking her case and the unpaid debt.

He said "I think they (Iran) are trying to use this as leverage to settle the historic debt for the Chieftain tanks and that's what they want settled. We obviously reject any linkage because we don't want to encourage hostage diplomacy and we want to follow the legal processes."

Initial Iranian media reports have linked today's release with the repayment of the UK's debt.

Foreign Secretary Liz Truss told the BBC that the debt to Iran was "legitimate" and that the government was "looking for ways to pay" it, despite continuing sanctions.

She did not link the issue to Ms Zaghari-Ratcliffe's release, but said that securing her freedom - and that of other dual national detainees - was "an absolute priority".

Ms Truss is expected to give more details in a statement later today.