
In short, it is the standard of proof required to be used by ECOs when the requisite documentary evidence to prove the applicant’s eligibility for UK visa being applied for is inadequate or lacking.
Let’s say the applicant is applying for a UK Spouse Visa from the Philippines. In such an application, the most genuine and convincing proof that the applicant for the UK Spouse Visa is legally and validly married to her British partner/sponsor here in the Philippines is of course the marriage certificate duly issued by the Philippine Statistics Authority (PSA). Hence, this certificate may be considered as a mandatory document.
However, as an example, what if the marriage certificate is not yet available because of some delay with the PSA but the Filipina wife needs to travel to the UK because her British husband is sick or figured in an accident.
In this case, the UK Spouse Visa application of the wife may be considered by the ECO as one falling under an EXCEPTIONAL COMPASSIONATE CIRCUMSTANCES because refusing it would cause an unjustifiable harsh circumstance. Menaing no one will take care of the husband in the UK if the application is refused. However, the fact remains that the applicant still has to prove she is legally married to her British husband/sponsor.
Under this scenario, if the marriage certificate cannot be acquired at the time the application is lodged and the application is filed without the very important certificate, the ECO does not have to refuse the application outright but may have to consider the application on the balance of probabilities. This is one of the discretions vested on the ECO.
Thus if he is satisfied, based on other documentary proof submitted by the applicant minus the marriage certificate, that the UK Spouse Visa applicant is genuinely and legally married to the British partner/sponsor, he may decide on the balance of probabilities that the applicant is probably qualified. This means that taking into account the other supporting documents which corroborate the fact that the couple is married to each other, and there is a more than 50% chance that the marriage is valid, coupled with the strength of the other documents taken as a whole, the application should be successful.
A number of secondary supporting documents to prove a valid marriage such as photos of the wedding, marriage licence, wedding invitation, email messages by the couple talking about the wedding and other minor proofs, taken together may be used by the ECO to consider whether the marriage is valid and the application should be accepted or rejected.

With respect to a refused application, however, a negative decision based on balance of probabilities would therefore mean that the applicant was not able to fully satisfy the ECO of his or her eligibility as a UK Spouse Visa applicant because of an absence of one or some very important and mandatory documentary evidence (marriage certificate in the example given above) and the secondary evidence attached with the application was not sufficient enough to convince the ECO that the applicant is probably qualified for the visa.
On the other hand, had the ECO been convinced that the secondary evidence were adequate enough to establish that the applicant is married, then on the balance of probabilities, the application should be successful and the applicant eligible for the UK Spouse Visa. This means that the ECO decided that the applicant is probably qualified for the UK Spouse Visa based only on the secondary evidence that was submitted.
It is worth mentioning therefore that BALANCE OF PROBABILITIES does not only apply to rejected applications but likewise pertains to successful applications. However, there is no need to cite that the application was approved based on the BALANCE OF PROBABILITIES as the grounds for approval of an application are not required to be mentioned anymore.
For refused applications, however, the grounds for refusal must specifically be stated by the ECO, and this is where the phrase BALANCE OF PROBABILITIES comes in.
ECO-Friendly therefore advises potential applicants to attach all mandatory documents to the application in order to have a strong chance of obtaining a positive result.
However, if by any chance, a particular mandatory document to prove an important fact in the application cannot be obtained and the applicant has no other choice but to just rely on secondary documents, they must ensure that all the secondary documents are sufficient and suitable enough to support and affirm the fact being asserted. This is so that on the balance of probabilities, the ECO will be satisfied that the applicant is probably qualified. This means that the ECO recognized and came to a conclusion of the fact that there was a 50% plus 1 that the applicant is probably eligible based on the documents that was presented.
Always take note that these secondary documents works as a replacement or substitute in the absence of the primary document and therefore should be compelling enough so that if ever the ECO makes a decision based on the balance of probabilities, he will be more inclined to issue a positive result because the applicant was able to satisfy his curiosity and validate the authenticity of the fact being alleged.