A former British army sergeant whose two sons are English rugby internationals is stuck in Fiji, prevented by immigration rules from returning to the UK to rejoin his wife as she undergoes cancer treatment.

Ilaitia Cokanasiga, who over almost 14 years in the armed forces served two tours in Iraq and one in Afghanistan, told the Guardian that his immigration difficulties had stopped him from travelling to see his 22-year-old son, Joe Cokanasiga, play for England in the World Cup in Japan last year. He is devastated at being stranded 10,000 miles away from his family, unable to support his wife as she waits for an operation on a brain tumour.

“I feel betrayed after what I did for the army and the country,” he told the Guardian. “I talk to my wife every other day, and she cries on the phone. She is very worried. I’m not there and she’s supposed to be having surgery. Everyone in the family is struggling to cope with the pressures of my wife’s poor health. I need to return to the UK to care for my wife.”

Joe Cokanasiga during a training session before the Rugby World Cup semi-final against New Zealand in Tokyo last October. Photograph: Mark R Cristino/EPA

Joe, whose younger brother Philip has played for London Irish and England at under-18 level, said he feels “really angry” about the situation. “Between my father, my brother and I, I believe we have served the United Kingdom in more ways than any ordinary British person,” he went on. “We have been made to feel that my father has done something wrong, when in fact it is the British army that has let him down. For me, this has been a really bitter pill to swallow.”

Ilaitia’s account echoes the situation faced by hundreds of Commonwealth-born army veterans who face serious immigration problems, despite long periods of service in the military. Some have been detained, others have had to leave the country, and many have lost their jobs after being told they had no right to be in the UK.

Earlier this week the Guardian revealed that a group of soldiers recruited from Fiji were taking legal action after they were classified as illegal immigrants following their discharge from the army.

Ilaitia Cokanasiga (far right) with his family. Photograph: Family handout

The Cokanasiga family had initially wanted to avoid publicity but were prompted to speak out after discovering how many other former soldiers face the same problem. Commonwealth veterans who have served more than four years are eligible for indefinite leave to remain, but the post-discharge immigration system is complex. Many assume that the process is automatic, since they have a stamp in their passport stating they are immigration exempt, and are unaware that the status ends when they leave the army.

“I had immense pride to have served in the British armed forces, and to have both my sons representing England in rugby,” Ilaitia, 48, said. “I now reflect on my army career with underlying bitterness and my pride at my sons’ rugby careers is tainted by the fact that the British government let our family down. I have never done anything unlawful in my life, so to be told that I did not have valid status in the UK was a real blow.”

The legal action against the Home Office and the Ministry of Defence launched last week accuses officials of a “systemic failure” to assist Commonwealth veterans with the difficult and expensive process of regularising their status. Once they are made aware of the problem, some have struggled to resolve it because Home Office application fees have risen from £1,051 in 2015 to the current £2,389.

Ilaitia was shocked to discover last year that the exemption stamp no longer gave him the legal right to remain in the UK. MoD rules require that the rules be explained to all non-British veterans in the period before discharge, but he said he received no guidance when he left in 2013.

Joe was alerted to his father’s immigration problems last summer, when he was returning from an England squad training session abroad. An immigration officer asked him how he had been granted indefinite leave to remain (ILR), and on hearing that it came through his father’s service in the army, told him: “I don’t think the visa your father had when he was in the army is valid any more.”

He was allowed to pass through immigration, but his father sought advice from Vinita Templeton of Duncan Lewis solicitors, a specialist in this area. She confirmed that the immigration exemption stamp in his passport – which reads “the holder... is not subject to any condition or limitation on the period of permitted stay in the United Kingdom’ – was now invalid, so he had no right to be living in the UK.

Home Office stamp granting permission to remain in the UK. Photograph: supplied

Ilaitia missed Joe’s appearance at the World Cup, because he had been advised that UK border guards might not allow him back in. However his daughter, Misi, was getting married in Fiji in December and he decided he could not miss her wedding. If he were to now return without the correct immigration status he would be in breach of UK law.

During the two months that Ilaitia has been stranded in Fiji he has met many other veterans who have been discharged from the army without receiving immigration guidance; many of them lost their jobs when their exemption stamps were not accepted as proof of the right to be in the UK and returned to Fiji because they could not afford the cost of the ILR application. Others have tried to make applications to return to the UK, but have been refused.

Ilaitia, who worked for National Rail after the army, now hopes to be granted leave to return by the Home Office swiftly. He said of his application: “I have been deterred by thoughts of whether an application would be successful. I also have worried about the impact my situation might have on my sons’ rugby careers if the public perceive that I have been in UK illegally.”

He said he felt “heartbroken” to have missed watching his older son play at the World Cup, and to be missing his younger son’s London Irish matches. “I risked my life for the army. When we were in Afghanistan our base in Kandahar was mortared every day for six months,” he said. A close friend who was also recruited from Fiji was killed during that tour. Both men had signed up in 1999 when around a thousand Fijians were taken on during an unprecedented recruitment drive on the island. “It was every boy’s dream to join the British army. I felt extremely proud,” Ilaitia said. “Now I feel angry.”

A government spokesperson said: “The government highly values the service of all members of HM forces, including Commonwealth nationals. The Ministry of Defence make clear to foreign and Commonwealth recruits into the forces the process by which they and their families can attain settlement in the UK, and the costs involved.”

Ilaitia’s lawyer, Vinita Templeton said: “Mr Cokanasiga’s situation highlights just how widespread this issue is, and how ex-servicemen can unwittingly remain unaware that their exemption stamp does not confer a valid right to remain in the UK after being discharged. This is a family that has benefited the United Kingdom hugely, but has been severely let down by the government.”