Sam Cataki, 36, who has lived in Neyland for the past five years, has also been told he can no longer work.
He lost his job at Milford Haven Port Authority on Wednesday and, with two young children aged one and five, said he fears for his family's future.
The Home Office has been contacted for comment.
Before he left the Army in 2013, Mr Cataki said he applied for citizenship in the UK but believes he was rejected because of three points on his driving licence.
"They say it doesn't count as 'good citizenship' which is one of the criteria," he explained.
He was given further leave to remain until his points cleared and told he would then be able to re-apply for citizenship.
"I applied again last year but that's when they told me it was void. They're telling me to re-apply for settlement [with] Armed Forces indefinite leave to remain but I'm still waiting," he said.
"I normally don't complain about anything but I have followed every procedure they've asked.
"After serving they should be giving more to ex-servicemen but it's like they have neglected and left me. They thank you for your service and then you're forgotten about."
Mr Cataki had been working in Milford Haven for two years and said the situation was also putting his wife Seini's visa status at risk.
"We really enjoy it here, we just want to work," he added.
"I've spent about Â£5,000 on applying for visas but without a job it will be difficult to afford applying again."
Stephen Crabb, MP for Preseli Pembrokeshire, has been assisting Mr Cataki since 2014.
In a message on Facebook on Friday, Mr Crabb said: "Lots of strong support from our local community for Sam Cataki over his visa application. He is a lovely guy and served in our Armed Forces for more than a decade.
"I have briefed the immigration minister and requested that the Home Office treats this as an urgent case. I have also asked for them to restore his right to work in the UK immediately.
"Having strong immigration rules is important but clearly the system should work better for our ex servicemen and women from Commonwealth countries."
A petition set up by the Neyland community in support of Mr Cataki has attracted nearly 20,000 signatures.
A Fijian father who served in the British Army for 12 years says he feels "neglected" after being refused UK citizenship.