Article 21

The respondent shall communicate its statement of defence in writing to the claimant and to each of the arbitrators within a period of time to be determined by the arbitral tribunal. The respondent may elect to treat its response to the notice of arbitration referred to in article 4 as a statement of defence, provided that the response to the notice of arbitration also complies with the requirements of paragraph 2 of this article.

The statement of defence shall reply to the particulars (b) to (e) of the statement of claim (art. 20, para. 2). The statement of defence should, as far as possible, be accompanied by all documents and other evidence relied upon by the respondent, or contain references to them.

In its statement of defence, or at a later stage in the arbitral proceedings if the arbitral tribunal decides that the delay was justified under the circumstances, the respondent may make a counterclaim or rely on a claim for the purpose of a set-off provided that the arbitral tribunal has jurisdiction over it.

The provisions of article 20, paragraphs 2 to 4, shall apply to a counterclaim, a claim under article 4, paragraph 2 (f), and a claim relied on for the purpose of a set-off.