Bailiff: (to the witness) Please raise your right hand. Do you swear to tell the truth, the whole truth, and nothing but the truth?
Witness: No.
What would happen? Unfortunately, in the limited research I did, I could not find a definitive answer.
Transcript adapted from here. Question inspired by this post.
That will amount to refusal to testify.
The judge will warn you that, for a summoned witness, a refusal to testify means contempt of court and you will be asked that question again. After a second "No" you go to jail for contempt of court, and the trial will most likely be adjourned until you make up your mind to answer "Yes".
A witness who attends court but who refuses to take the oath or affirmation, or who improperly refuses to give evidence, is liable to be fined or imprisoned.
In the magistrates’ court, s.97(4) Magistrates’ Courts Act 1980 enables the court to impose up to one month’s imprisonment or a fine up to £2500 for any of the following acts committed by a witness without just excuse:
refusal to take the oath or affirm;
refusal to give evidence;
refusal to produce a document or other item.
In the Crown Court, the same refusals will amount to contempt of court punishable by up to 2 years’ imprisonment or an unlimited fine.
Defence Barrister UK: Witness attendance at court but failure to testify
Per the Law Reforms Commission of Ireland,
Finally, it may be noted that a witness who, without lawful excuse, refuses to be sworn or to make an affirmation is guilty of contempt in the face of the court and may be fined and imprisoned
THE LAW REFORM COMMISSION - REPORT ON OATHS AND AFFIRMATIONS (1990).
In Portuguese law
Refusing to take the oath is equivalent to a refusal to give evidence; both are punishable as contempt of court unless justified, should the judge charge the person accordingly.
What happens if I negatively answer the court oath regarding the truth?
You would be given few more opportunities to rectify, very likely with the judge pressing you to answering properly. But a persistent negative will put you in [direct,] civil contempt of court for disrupting the proceedings. It is called direct because the seemingly vexatious conduct is in the presence of the judge and is considered an affront to the "dignity" of the court.
In Germany there are fairly wide-ranging reasons to legally refuse to testify; close relatives and spouses don't need to help prosecuting their loved ones and it is legally impossible to put an accused under oath.
Even if not accused, one can legally refuse to testify (let alone under oath) if that statement would be self-incriminating.
But your question surely concerns situations where none of the above applies.
Par. 70 of the German code of criminal procedure specifies, equally for witnesses who refuse to testify at all, or refuse to testify under oath:
Certainly, as always, the implicit Basic Law rule of proportionality ("Verhältnismäßigkeit") must be considered. That principle limits all government action to be as mild as possible when regulating peoples' lives. It is similar to the Eighth Amendment, but more comprehensive, and immediately applicable law.
In this particular case it certainly means that the enforcement measures cannot qualitatively exceed the expected punishment/compensation. If the suit is about stealing an apple it would probably be illegal to arrest an unwilling witness, even if we consider that there is additionally a general interest in our social order to pursue and sanction crimes. If, on the other hand, it is about murder, and the statement is decisive, the court will likely exhaust its available enforcement measures.
As an aside, there is an amusing scene in the German film Stammheim where a witness defies the judge's order to testify. He says "Ich muss hier gar nichts" ("I don't have to do a thing here") and is lectured ("please inform him about the consequences") and threatened with enforcement arrest and fines. I'm not sure about the witness's identity but I must presume that he already serves a long sentence for his "political fight" (i.e., terrorist activity) so that the threats of enforcement are not the sharpest blade in the judge's arsenal.
An interesting detail is that at that time (1977, probably) apparently the court could refrain from demanding an oath if the defendants agreed. That "Ziffer 5" must since have been removed from §61 of the Code of Criminal Procedure (StPO).
