According to the established case law of the boards of appeal, filing new facts, documents and evidence would be considered to be in due time, if the filing was occasioned by an argument or a point raised by another party or in the appealed decision and could not have been filed before under the circumstances of the case. However, in order not to deprive the other parties of their right to verify the new evidence or to prevent the board from ensuring that the proceedings are conducted expeditiously, such facts and evidence have to be submitted once they are available and once it has become clear that they were relevant (T 201/92; T 951/91, OJ 1995, 202; T 502/98; T 730/07; T 320/08; T 976/10). Late filing is justifiable if it is an appropriate and immediate reaction to developments in the previous proceedings (T 855/96). Late filing is also admissible in order to support the reasoning to date (T 561/89) and to prove claimed common general knowledge, if challenged (T 106/97, T 1076/00).
The delay is of less importance if the party was reacting to a finding in the contested decision (T 101/87, T 1511/06). An appellant who has lost the opposition proceedings should be given the opportunity to fill the gaps in its arguments by presenting further evidence in the second instance (T 406/09).
In T 49/85 the board stated that a reference filed by an opponent for the first time with its statement of grounds was not submitted in due time unless representing the effective counter evidence to a newly emphasised reason given in the decision (see also T 172/85 and T 561/89). It is however within the discretion of a board of appeal pursuant to Art. 114(1) EPC to admit such a document into the proceedings in view of its relevance (T 339/08). The same applied if the party reacted to its opponent's submission, e.g. in T 705/90, where documents I to N were cited by the appellant in its observations in support of an argument presented in response to the respondent's statement of grounds.
In T 1045/08 the board stated that where a party files new requests in appeal proceedings the other party must have the possibility of responding to these including the filing of new prior art documents, particularly when some of the amendments to the claims of the requests are based on features taken from the description of the patent. In this situation the concept of "late filed" must be considered relatively since the documents could not have been filed earlier because the party could not know the content of the future requests of the other party.
In T 1817/15 the board considered that where evidence is belatedly filed on appeal as an objectively appropriate reaction to the impugned decision, such evidence should be sufficiently relevant to the outcome of the assessment of the patentability of the subject-matter claimed. However, a final decision on the relevance of these documents should not be given in the event that the case is then remitted to the first-instance department for further prosecution upon admission. Otherwise, the latitude of examination by the first-instance department would be unduly restricted (see e.g. T 736/99). In other words, it would suffice in such cases to demonstrate that it is prima facie not unlikely that the admission of such fresh evidence could change the outcome of the assessment of the first-instance department.