The 34-year-old, from Harrow, west London, is accused of 37 charges – including rape, false imprisonment and kidnap – relating to 11 women and children.
McCann, who denies the accusations against him, had chosen not to be present at the Old Bailey as the prosecution outlined its case against him.
But as it closed its arguments, the jury was told he had changed his mind.
Mr Justice Edis said he was sorry that they had been “kept in the dark and kept hanging around”.
He added: “I told you some time ago that the defendant had decided not to attend his trial but that he would be able to do that if at any time he changed his mind.
“Obviously, the time has come because the prosecution has all but closed its case, if he wants to, the defendant can give evidence.
“Today, the court has received information that he does now wish to attend his trial.
“That is something that cannot simply be arranged immediately and therefore we have been considering arrangements for that during the course of this morning.”
The case is expected to resume on Tuesday at 11am.