Mel made the admission while filming for Piers Morgan's Life Stories on Friday

The 43-year-old, who is openly bisexual, revealed it was a one-off sexual encounter

Shortly after the interview, it was claimed 'Scary Spice' made a panicked phone call to Geri to apologise

It is alleged that the racy confession will anger Geri's husband Christian Horner as she has 'worked hard to reinvent herself as a perfect Stepford wife'

The Spice World - 2019 tour kicks off on May 24 in Dublin with dates in Cardiff, Manchester, Coventry, Sunderland, Edinburgh, Bristol and London

The Spice Girls reunion tour is feared to be jeopardy after Mel B revealed she had sex with Geri Horner during the height of their fame in the nineties.

The openly bisexual singer, 43, made the shock admission while filming for Piers Morgan's Life Stories on Friday, cheekily telling the broadcaster Geri had 'great boobs' and adding: 'It just happened and we just giggled at it and that was it.'

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However, the saucy details of their night of passion has already reportedly caused great upset, with sources revealing Mel made a frantic call to Geri amid claims the confession upset her husband Christian Horner.

Night of passion: Mel claimed that she has had sex with Geri Horner, 46, during the height of their Spice Girls fame in the nineties

Speaking of her time in the girlband - which were together from 1994 to 2000 and are now set for a reunion tour - Mel was clearly not in the mood to hold back as she detailed her relationship with Geri, explaining: 'We were best friends. It just happened.

'She (Geri) is going to hate me for this because she is all posh in her country house and her husband, but it's a fact. It just happened and we just giggled at it and that was it. She had great boobs.'

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When Morgan enquired how frequently they slept together, the 43-year-old, real name Melanie Brown, said ‘Oh stop it, you pervert’, before eventually confirming there was a solitary sexual encounter.

'It just happened': The openly bisexual singer made the shock claim while filming for Piers Morgan's Life Stories, cheekily telling the broadcaster she had 'great boobs' (Pictured 1997)

'Struggles to keep her mouth shut: Sources revealed Mel made a frantic call to Geri to apologise straight the interview amid claims the confession may upset her husband Christian Horner (pictured)

Piers delightedly expressed: 'This is a bombshell. A Spice Girls bombshell. You have never admitted this before have you? People are going to be fascinated!'

Mel responded: 'Yeah and I have said it now. All done. She is going to kill me and so is her husband.

'Hopefully when Geri gets asked that, which hopefully she will after this, she won't deny it, because it was just a thing, a fun thing,' she stated.

Meanwhile, her bandmate Melanie Chisholm – Mel C – who was in the audience looked shell shocked by the revelation, admitting: 'This is all new to me.’

Although Geri hasn't commented on Mel's claims, she previously hinted at something more than friendship with her pal also during an unearthed interview with Piers Morgan in 2010.

'We just giggled and that was it': Speaking of her time in the girlband Mel was clearly not in the mood to hold back as she detailed her relationship with Geri

Piers delightedly expressed: 'This is a bombshell. A Spice Girls bombshell. You have never admitted this before have you? People are going to be fascinated!'

Discussing Mel's sensational figure, Geri told Piers: 'She's absolutely gorgeous, even more gorgeous in real life. She has great boobs... she's got an amazing bottom.'

However, it appeared that Mel began thinking twice if the saucy encounter was the correct thing to reveal to audiences, as The Sun claim she made a panicked phone call to Geri to apologise.

In light of the alleged call, sources told the publication the supposed root of her alarm: 'The girls all know Mel struggles to keep her mouth shut but Mel C who was in the audience was visibly horrified when it all came out.

'She's going to kill me!' It appeared that Mel began thinking twice if it was the correct thing to reveal to audiences, as The Sun claim she made a panicked phone call to Geri to apologise

'The tour is just two months' away they don't want anything jeopardising it, but there were real fears this would upset Geri's husband and causes serious issues.

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'Since marrying Christian, Geri has worked hard to shake off her old playgirl image and sort of reinvent herself as this perfect Stepford Wife. Mel immediately rang Geri to try to repair the damage.'

Geri married Red Bull racing tycoon husband Christian Horner who she shares son Montague, two, with and also has daughter Bluebell, 12, with ex Sacha Gervasi.

Cheeky: Although Geri hasn't commented on Mel's claims, she previously hinted at something more than friendship with her pal also during an unearthed interview with Piers Morgan in 2010

Mel B, meanwhile divorced film producer Stephen Belafonte in 2017 after a turbulent ten-year marriage, and detailed a chaotic life of sex and drugs in a recent autobiography.

And she has justification for her concern about how Geri will react as the pair are known to have a fractious relationship.

It will come under more stress when the band – minus Posh Spice Victoria Beckham – begin rehearsals for their UK tour in a fortnight.

Previous antics: Geri is said to be furious as she has 'worked hard to shake off her old playgirl image and sort of reinvent herself as this perfect Stepford Wife'

Prior to finding love with Christian, Geri dated a host of celebrities after shooting to fame with the Spice Girls in the nineties.

Geri began dating Chris Evans in 1999 for a short while before he married Billie Piper and Geri began seeing man-of-the-moment Robbie Williams until 2001.

Her dating history around that time also includes limp Bizkit frontman Fred Durst, and two members of Blue, Duncan James and Lee Ryan, who are rumoured to have enjoyed a fling together.

In 2005 Geri enjoyed a short-lived romance with David Walliams over the course of just six weeks, before attempting to settle down with Fabrizo Politi in 2008.

However just four months after he popped the question they called it quits.

Other famous flings include Russell Brand, who she was spotted with during the time of the 2012 Olympics in London.

The publication also claims that Mel and Geri's night of passion supposedly happened when they girls moved into a shared house in Maidenhead in the mid-nineties.

Love: Mel has been open about her sexuality and previously admitted to having a long-term romance with a woman. She also endured a bitter split from ex Stephen Belafonte in 2017

Impressive: The Spice Girls sold 85 million records over their career, scoring nine No 1 singles including Wannabe and 2 Become 1, before splitting in 2000

MailOnline has contacted representatives for Mel B and Geri Horner for further comment.

Mel has been open about her sexuality and previously admitted to having a long-term romance with a woman. She also endured a bitter split from ex-husband Stephen Belafonte in 2017.

During her rise to fame Mel dated Fjolnir Thorgeisson between 1996 and 1998, before she met Dutch dancer Jimmy Gulzar who Mel began seeing during the SpiceWorld tour.

They got married in September 1998, and Mel changed her stage name to Mel G.

Exciting! The Spice World - 2019 tour, without Victoria, kicks off on May 24 in Dublin with dates in Cardiff, Manchester, Coventry, Sunderland, Edinburgh, Bristol and London

During the years in the wake from her split with Jimmy, Mel dated British actor Max Beesley and film director Christine Crokos.

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Mel hit the headlines in 2006 when she began dating A-list actor Eddie Murphy, when it was revealed that they were expecting a baby together, Angel, now 11.

The Spice Girls sold 85 million records over their career, scoring nine No 1 singles including Wannabe and 2 Become 1, before splitting in 2000.

The Spice World - 2019 tour kicks off on May 24 in Dublin with dates in Cardiff, Manchester, Coventry, Sunderland, Edinburgh, Bristol and London.

Victoria will not be joining the girls on tour, with the star stating she has her hands full running her 'fashion empire'.