Border inspectors visited the Warsaw Visa Section in Poland, a huge processing hub that deals with 56,000 applications a year from foreign nationals based in 12 countries including Ukraine, Bulgaria, and Romania among others.
But inspectors found that 81% of all "other visitor" applications between August 2012 and July this year were from Ukraine.
Intelligence officers, entry clearance staff and managers told inspectors that a higher proportion of applications in the Ukrainian capital Kiev were supported by "forged or counterfeit documents", making the location a "high-risk post".
Despite this, an application for entry clearance in Ukraine was found to be much less likely to be refused than an application made in Austria or Latvia - considered a much lower risk of fraudulent behaviour.
Furthermore, intelligence staff told inspectors additional risk-based verification checks by entry-clearance staff were only being conducted in 4% of applications.
The report said: "We were surprised to find that the refusal rate for Kiev was significantly lower than for other posts in the region.
"We would have expected a post which the risk profile indicated was a high risk to have had a refusal rate exceeding that of lower risk."
A Home Office spokeswoman said: "We are satisfied that no individual was issued with a visa when they should not have been."