A Scots Guard who was unveiled by the British Army as its first female on the front line after changing sex is having an 'affair' with another soldier's wife, it was claimed last night.

The Mail on Sunday understands Guardsman Chloe Allen, 26, is living with married teaching assistant Sophie Gray, who helped the soldier to apply her make-up soon after telling military chiefs that she wanted to live as a woman.

The rifleman, who has guarded the Queen and marched at Trooping the Colour, made headlines in 2016 when the Ministry of Defence proudly announced she would continue in a combat role as a female, having served as a man for four years.

In a series of TV interviews, Chloe, from Penrith, Cumbria, said she was honoured to become the first woman in Army history to be allowed to fight on the front line.

Guardsman Chloe Allen(pictured left), 26, is living with married teaching assistant Sophie Gray(pictured right), who helped the soldier to apply her make-up soon after telling military chiefs that she wanted to live as a woman

But now The Mail on Sunday has been told Sophie's devastated husband intends to name Chloe in divorce papers citing his wife's infidelity as the reason for the breakdown of their marriage.

The legal move will embarrass top brass, who in 2016 used Chloe's decision to transition from male to female to showcase the Army's commitment to diversity and equality.

At the time, Commander of the Field Army, General Sir James Everard, applauded her courage and wished the 'trendsetter' every success.

The MoS understands Sergeant Mark Gray, 37, who was with Sophie for ten years, spoke about Chloe's relationship with his wife to his superiors, as under military law soldiers are not supposed to have affairs with other troops' partners.

But according to sources, senior officers declined to take action against Chloe.

Top brass were apparently more concerned about protecting her from a possible backlash from guardsmen angry about her affair with their comrade's wife.

Sgt Gray, from Reading, previously served in the Scots Guards and has friends in Chloe's battalion. Colleagues were concerned about the impact of his wife's new relationship on his emotional welfare.

A Guards source said: 'Mark is distressed and feels Chloe played him for a fool by having an affair with Sophie while maintaining that Sophie was only helping her learn how to present herself as a woman and wear make-up.

The Army championed Chloe (pictured) as its first transgender frontline soldier in September 2016, two months after a historic rule change which allowed women to serve in combat units for the first time

'Had Mark shacked up with another guardsman's wife, he'd have been charged with breaching disciplinary rules and been in deep trouble, whereas nothing appears to have happened to Chloe despite her affair with Mark's wife.'

The Army championed Chloe as its first transgender frontline soldier in September 2016, two months after a historic rule change which allowed women to serve in combat units for the first time.

At the same time she also began hormone therapy, the first stage of the sex-change process.

Chloe, who joined the Scots Guards as Ben Allen in 2012, decided to transition after being caught cross-dressing before she guarded a Royal palace.

In an interview arranged by the MoS she said the incident had been a 'blessing in disguise' because it prompted her to 'get on and deal with it'.

Chloe is now living with Sophie in Swindon, after Sophie moved out of the married Army quarters she previously shared with Sgt Gray in Aldershot.

The MoS understands they began a relationship two years ago after being introduced by mutual friends.

Chloe, whose Army job is to drive a 26-ton Mastiff armoured vehicle, would visit the Grays' family home and sit in the kitchen as Sophie applied her lipstick and eyeliner.

According to sources, Sgt Gray began to fear there might be more to his wife's friendship with Chloe. But when he confronted her, Sophie denied they were involved in a physical relationship.

Chloe (pictured), who joined the Scots Guards as Ben Allen in 2012, decided to transition after being caught cross-dressing before she guarded a Royal palace

Sgt Gray was apparently unconvinced and raised the situation with a welfare officer. Sources say Sgt Gray felt Chloe's conduct breached the Army's strict code, known as the 'service test', which forbids extramarital affairs. Troops found guilty of such misconduct can be kicked out of their regiments.

The MoS understands the issue was subsequently brought to the attention of Chloe's then commanding officer, Lieutenant-Colonel Rupert Kitching.

An Army source explained: 'Scots Guards commanders gave Chloe their full support and stressed they wanted her to remain in the regiment whatever was happening in her private life and with her transition from man to woman.

'They favoured handling the matter informally, a decision made easier because Sgt Gray was no longer in the Scots Guards.

'On that basis they decided a relationship between Chloe and Sgt Gray's wife would not cause a significant fallout between Scots Guards soldiers or potentially undermine the regiment's operational effectiveness.'

Sgt Gray is said to have returned from a four-month deployment to the Falklands to find Sophie had left their family home and taken all her belongings with her.

MoD sources said Chloe had recently transferred from the Scots Guards to the Military Provost Guards Service, which is responsible for security at military bases.

Last night, Chloe issued a statement through the MoD saying: 'This story is untrue.

'I am not in a relationship, nor am I living with anyone.'

However, in recent weeks Chloe has spent several nights at Sophie's two-bedroom flat which she shares with her two children. Chloe has also taken Sophie's children to and from school on multiple occasions.

Chloe took delivery of a king-size bed and mattress at the property last month, and on the same day bought fresh bedding from a local shop.