Alistair Smith, from Leconfield, near Beverley, is a bombardier with the Royal Artillery and has already missed a tour of duty in Iraq because of the row and will now miss further overseas service in the Falklands.

Details of the case have emerged at a time when the Army is under increasing pressure to meet its foreign commitments, with troops still deployed in Iraq and little prospect of change in Afghanistan.

He is a Zimbabwean national who left the country for London when his father's farm was seized by Robert Mugabe's war veterans in 2000, joining the Army two years later.

Zimbabwe refused to recognise his citizenship, meaning he could not get his passport renewed when it expired almost four years ago and since then he has made several unsuccessful applications for British nationalisation and a passport.

Now the 30-year-old has been told by the Home Office he must take a UK citizenship test, answering questions about life in this country before a further application can be considered.

That is despite his success in UK exams and support from his commanding officer who wrote to the authorities on his behalf .

Bombardier Smith has also trained at the Royal Military Academy at Sandhurst, although he withdrew for personal reasons.

He says his lack of operational experience is now holding back his ambition to gain promotion to sergeant and is damaging the effectiveness of his unit.

His MP, Graham Stuart, described the UK immigration system as "a farce and a shambles" and is demanding urgent answers from the Home Office.

"Hundreds of people who don't contribute appear to be treated with kid gloves, while someone who is prepared to put his life on the line for his country and clearly wants to contribute is prevented from doing so by ill thought-out imposition of the rules," he said.

"I have asked the Home Office for an urgent response."

The Home Office refuses to comment on individual cases and although sources suggest alternative documents could be available to allow soldiers to serve abroad, they had not been offered to Bdr Smith.

He said: "I have to stand back and watch my guys, who I work and live with, while they go out of the door. It's really demoralising."

The first application Bdr Smith made for UK citizenship was rejected because he was deemed to have used the wrong criteria to make his claim and a second fell foul of immigration rules which say applicants must take the test.

He appealed but was only told the Secretary of State had ruled against him when he had no time to sit the test.

The soldier is now expecting to take it next month, when he should have been involved in training for the Falklands tour, meaning even if citizenship is granted he will have to miss the operation.

The Ministry of Defence said it has had no involvement in the issue.