Cabinet minister Liam Fox has said people should not overreact to "slips of the tongue", after a verbal gaffe by Boris Johnson led to a British citizen being threatened with a longer prison term in Iran.

Dr Fox was attempting to defend the Foreign Secretary after incorrect comments he made in Parliament led to Nazanin Zaghari-Ratcliffe being hauled in front of an Iranian court and told her sentence may be doubled to ten years.

Mr Johnson was due to call his Iranian counterpart on Tuesday to undo damage after telling a committee hearing that Ms Zaghari-Ratcliffe worked in Tehran teaching journalism before she was arrested and jailed for spreading propaganda - despite a central part of her defence being that she had never done so.

He has failed to make a public apology or admission that his words were incorrect, as Shadow Foreign Secretary Emily Thornberry called for him to resign if it turns out that charity worker Ms Zaghari-Ratcliffe is imprisoned for longer.

Speaking to BBC Radio 4's Today programme, Dr Fox said: "We all make slips of the tongue, we’ve got to be very careful that we are not overreacting to this in a way that is going to give the Iranians [an excuse to extend the sentence]."

He also said: "I think it's important we keep this in perspective. First of all this is a UK citizen being held on extreme and unproven charges in an entirely unacceptable way. It’s right that we constantly remind people that her detention is utterly unacceptable in an international way .

"The point the Foreign Secretary was making was that this arrest and detention of a UK citizen is not acceptable."

UK news in pictures Show all 50 1 /50 UK news in pictures UK news in pictures 18 September 2020 A model presents a creation during the Bora Aksu catwalk show at London Fashion Week 2020 Reuters UK news in pictures 17 September 2020 World kickboxing champion Carl Thomas during his attempt to run a marathon while pulling a plane at Elvington Airfield near York. The attempt is raising funds for Ollie's Army Battling Against Battens, an organisation campaigning to raise GBP 250,000 to fund a clinical trial aimed at saving the sight of children with CNL2 Batten Disease PA UK news in pictures 16 September 2020 Labour deputy leader Angela Rayner speaking during Prime Minister's Questions UK Parliament/AFP/Getty UK news in pictures 15 September 2020 People enjoying the autumn sunshine as they punt along the River Cam in Cambridge PA UK news in pictures 14 September 2020 Early morning light bathes the skyscrapers of the City of London, at the start of a week in which the UK is expected to bask in temperatures of more than 30 degrees PA UK news in pictures 13 September 2020 England celebrate after they dismissed Australia's Alex Carey to win the second ODI match of the series at Emirates Old Trafford, Manchester PA UK news in pictures 12 September 2020 Protesters outside BBC Broadcasting House in central London, as marches and rallies form across the country calling for a 15% pay rise for NHS workers and an increase in NHS funding PA UK news in pictures 11 September 2020 An empty migrant dinghy floats off the beach at St Margaret's Bay after the occupants landed from France in Dover Getty UK news in pictures 10 September 2020 A view of small boats thought to be used in migrant crossings across the Channel at a storage facility in Dover, Kent PA UK news in pictures 9 September 2020 EU's chief negotiator Michel Barnier, left, arriving from the Eurostar with EU Ambassador to the UK, Portuguese diplomat Joao Vale de Almeida at St Pancras International railway station, London, for the latest round of the negotiations on a free trade deal between the EU and the UK PA UK news in pictures 8 September 2020 Dawn over Coquet Island, a small island off Amble on the Northumberland coast PA UK news in pictures 7 September 2020 A hovercraft arrives to Southsea, Hampshire from the Isle of Wight PA UK news in pictures 6 September 2020 Forensics officers near the scene of multiple reported stabbings in Birmingham Reuters UK news in pictures 5 September 2020 Anti-migrant protesters demonstrate in Dover against immigration and the journeys made by refugees crossing the Channel to Kent PA UK news in pictures 4 September 2020 Activists take part in a demonstration against the HS2 hi-speed rail line outside the Department of Transport AFP via Getty UK news in pictures 3 September 2020 Peter Baker, who plays Trigger in the musical version of Only Fools and Horses, sweeps the stage of the Theatre Royal Haymarket in London, after observing a 15 minute silence to show solidarity with those in the theatre industry that have lost their jobs due to the coronavirus pandemic PA UK news in pictures 2 September 2020 Kadie Lane, right, 11, and Brooke Howourth, 11, hug on their walk to Marden Bridge Middle School in Whitley Bay, Tyne and Wear, for their first day of term, as schools in England reopen to pupils following the coronavirus lockdown PA UK news in pictures 1 September 2020 Extinction Rebellion protesters sitting outside The Houses of Parliament in Westminster, London PA UK news in pictures 31 August Surfers at Long Sands Beach, Tynemouth PA UK news in pictures 30 August Black Lives Matter protesters march through Notting Hill in London in the first Million People March EPA UK news in pictures 29 August A protester reacts as she demonstrates against the lockdown and use of face masks, amid the coronavirus disease outbreak, outside Downing Street in London Reuters UK news in pictures 28 August Caribbean soca dancers display their costumes as they promote the first ever digital Notting Hill Carnival, following the cancellation of the normal Carnival festivities due to the continued spread of the coronavirus disease, in London Reuters UK news in pictures 27 August Father and son team Chris and Sam Milford from historic building conservation specialists WallWalkers begin restoration work on the spire of Norwich Cathedral, which stands at over 312ft high. The first known spire was completed in 1297 PA UK news in pictures 26 August Giant waves at Seaham in County Durham, as the bad weather continues PA UK news in pictures 25 August An assistant at the Wallace Monument cleans the case which houses the William Wallace sword in the Hall of Arms room at the monument near Stirling as they prepare to re-open PA UK news in pictures 24 August Restored World War Two landing craft LCT 7074 is transported from from the Naval Base in Portsmouth to its final resting place at the D-Day Story at Southsea PA UK news in pictures 23 August Jenny Nguyen and Tony Cao, from Vietnam, pose for wedding photos on Tower Bridge in London, as it remains closed to vehicles after it was stuck open on Saturday due to a "mechanical fault". The landmark's Twitter account confirmed only pedestrians and cyclists could use it on Sunday morning PA UK news in pictures 22 August England's Zak Crawley hit 267, joining the exclusive Double Hundred club, on day two of the Third Test match against Pakistan at the Ageas Bowl, Southampton PA UK news in pictures 21 August Harri Teale gathers lavender during the annual harvest on the Wolds Way Lavender farm near Malton in North Yorkshire PA UK news in pictures 20 August Parents and a student react after checking GCSE results at Ark Academy in London Reuters UK news in pictures 19 August Tate Modern workers hold a strike outside the gallery in London, to protest the institution's announcement that it would cut more than 300 jobs from its commercial arm, Tate Enterprises PA UK news in pictures 18 August Two rescued brown bear cubs, Mish (left) and Lucy, cool off in a pool after arriving at their new home with the wildlife conservation charity Wildwood Trust in Herne Bay, Kent. The orphaned pair, who have been living in a temporary home in Belgium since they were found abandoned and alone in a snowdrift in the Albanian mountains, will be acclimatised to their new life in the country before moving to a permanent home PA UK news in pictures 17 August A level students celebrate outside the Department for Education in London after it was confirmed that candidates in England will be given grades estimated by their teachers, rather than by an algorithm. The government U-turn comes just days after Education Secretary Gavin Williamson vowed there would be "no U-turn, no change. PA UK news in pictures 16 August Wasp players take a knee as Northampton Saints stand prior to kick-off in their Premiership match at Franklin's Gardens PA UK news in pictures 15 August Piper Colour Sergeant Lil Bahadur Gurung attends the VJ Day National Remembrance event, held at the National Memorial Arboretum in Staffordshire, Britain Reuters UK news in pictures 14 August People including students hold placards on Whitehall outside Downing Street as they protest against the downgrading of A-level results. The government faced criticism after education officials downgraded more than a third of pupils' final grades in a system devised after the coronavirus pandemic led to cancelled exams yes AFP via Getty UK news in pictures 13 August Benita Stipp (centre) and Mimi Ferguson (left) react as students at Norwich School receive their A-Level results PA UK news in pictures 12 August 2020 A train derailment near Stonehaven has left three people dead. Driver Brett McCullough, conductor Donald Dinnie, and a passenger were killed when the 6.38am Aberdeen to Glasgow Queen Street service crashed amid heavy rain and flooding BBC UK news in pictures 11 August 2020 A woman hydrates in the sun after open water swimming at the West Reservoir Centre in north London Tolga Akmen/AFP via Getty UK news in pictures 10 August 2020 Prime Minister Boris Johnson takes part in an archery session as he visits Premier Education Summer Camp at Sacred Heart of Mary Girls' in Upminster Reuters UK news in pictures 9 August 2020 People cycle through Cambridge as the heatwave continues in Britain EPA UK news in pictures 8 August 2020 Healthcare workers take part in a protest in London over pay conditions in the NHS Getty UK news in pictures 7 August 2020 Emergency services make their way along the seafront on Bournemouth beach in Dorset on one of the hottest days of the year PA UK news in pictures 6 August 2020 Alison Murphy poses for a picture by husband Peter as she walks through a field of sunflowers in Altrincham, Cheshire PA UK news in pictures 5 August 2020 Pakistan's Abid Ali being bowled by England's Jofra Archer during day one of the First Test match at the Emirates Old Trafford, Manchester PA UK news in pictures 4 August 2020 The 'Timbuktu tumblers' from Kenya perform their balancing act on the Southsea waterfront as Zippos Circus reopens in Portsmouth Rex UK news in pictures 3 August 2020 Pelicans interact with a visitor in St James's Park in London PA UK news in pictures 2 August 2020 Lewis Hamilton drives with a puncture towards the finish line to win the Formula One British Grand Prix at Silverstone POOL/AFP via Getty UK news in pictures 1 August 2020 Arsenal's Pierre-Emerick Aubameyang celebrates with the trophy and teammates after winning the FA Cup, as play resumes behind closed doors following the outbreak of the coronavirus disease Pool via Reuters UK news in pictures 31 July 2020 People enjoy the sunny weather at a Bournemouth Beach Reuters

Mr Johnson’s appearance at a select committee last Wednesday saw him insist Ms Zaghari-Ratcliffe is innocent and condemn her treatment, but he then added "she was simply teaching people journalism as I understand it."

On Saturday, the first Iranian working day after Mr Johnson’s erroneous remarks, Ms Zaghari-Ratcliffe was taken before hardline judge Abolghassem Salavati, to be told there were new accusations against her, under a new charge of “spreading propaganda against the regime”

She was threatened with another five years in prison – on top of her existing five-year sentence.

The Iranian High Council’s website also now carries a story headlined "UK confirms Nazanin Zaghari-Ratcliffe was not in Iran for Holiday", stating: "[Johnson’s] statement shows that Nazanin had visited the country for anything but a holiday. For months it was claimed that Nazanin is a British-Iranian charity worker who went to see her family when she was arrested.

"Mr Johnson's statement has shed new light on the realities about Nazanin, which has been strongly denied previously by both her family and Human Right activists such as [Iranian Nobel Peace Prize winner] Shirin Ebadi.”

Jailed in Iran: Richard Ratcliffe calls on the government to bring Nazanin home

Pressure is mounting on the Foreign Secretary to repair the damage. In a letter to Mr Johnson Labour's Ms Thornberry said his comments had revealed "a fundamental lack of interest or concern for the details of Nazanin’s case and the consequences of your words."

She said: "While your previous gaffes in the role of Foreign Secretary may have seriously damaged this country's interests abroad, and caused grave offence to our international partners, this is - I believe - the first time that one of your comments has directly harmed the interests and prospects of a British national who instead should have been entitled to expect your support and aid."

Foreign Secretary Boris Johnson is under pressure over his gaffe (WPA Pool/Getty Images) (Getty)

The Labour frontbencher said responsibility for the crisis is "your alone", accusing him of committing a "gross error" and being guilty of "ineptitude".

She added: "In the event that your actions have indeed cause irreparable harm to Nazanin's prospects of freedom and result in her sentence being lengthened, I hope and trust that you will take full responsibility for that, in both a moral and political sense, and consider your position accordingly."

Ms Zaghari-Ratcliffe’s husband, Richard, has also claimed that she was accused of a plot to topple the Iranian government and has insisted that she is innocent.

Nazanin Zaghari-Ratcliffe is imprisoned in Iran (PA)

He said he had no doubt the fresh charges were triggered by the Foreign Secretary’s remarks, adding: "The worst thing the Foreign Secretary could do is to now suddenly go quiet and to create this problem without making any clarifications.

“This is an error that urgently needs to be corrected, otherwise there is a real risk it will be used to add years to Nazanin’s jail sentence.”

Stressing that Mr Johnson needed to make a statement to Parliament as well as speaking privately to his Iranian counterpart, Mr Ratcliffe said: “There is a limit to the Iranian foreign ministry’s power in this.

“As well as a private conversation, there needs to be a clear public statement of equal status to the initial error, so that things are done in a way that the Iranian judiciary has to acknowledge.”

Mr Ratcliffe told The Independent that while he welcomed Dr Fox’s acknowledgement that his wife was being held unfairly, the International Trade Secretary’s suggestion that people shouldn’t overreact to a slip of the tongue was “unfortunate”.

“That was a flippant statement,” said Mr Ratcliffe. “It is clear that the Iranian judiciary is using Boris Johnson’s error to build a case against Nazanin – let us not be too cavalier about the consequences.”

Mr Ratcliffe had earlier explained that that his wife had never actually taught journalists at any point in her career.

Instead in her first graduate job eight years ago, when she was an assistant at BBC Media Action, the corporation’s international development charity, she had simply made travel arrangements for tutors on an international journalism course.

Ms Zaghari-Ratcliffe’s current employer, the Thomson Reuters Foundation, has also confirmed that the charity worker was not teaching journalists, with its chief executive Monique Villa saying: “Nazanin is not a journalist and has never trained journalists at the Thomson Reuters Foundation."

Mr Johnson is yet to comment on the controversy but the Foreign Office issued a statement apparently accepting that his remarks had raised the threat of a longer prison term.

A spokesman said: "Last week’s remarks by the Foreign Secretary provide no justifiable basis on which to bring any additional charges against Nazanin Zaghari-Ratcliffe.

"While criticising the Iranian case against Mrs Zaghari-Ratcliffe, the Foreign Secretary sought to explain that even the most extreme set of unproven Iranian allegations against her were insufficient reason for her detention and treatment."

It added that Mr Johnson would be telephoning his Iranian counterpart to "raise again his serious concerns about the case and ensure his remarks are not misrepresented".

Ms Zaghari-Ratcliffe, 38, was separated from her 21-month-old daughter Gabriella when she was arrested as she tried to return home to London after a two-week holiday.