The International Criminal Court has told Kenya's Deputy President William Ruto he must attend most of his trial on charges of crimes against humanity.
Prosecutors had appealed against an earlier decision which would have let him spend most of his time in Kenya.
While he must appear at most of his trial, the court ruled he can be excused on a "case by case" basis.
Mr Ruto's lawyers argued he was needed in Kenya after the attack by Islamist militants on the Westgate centre.
He denies responsibility for post-election violence in 2007 and 2008. An estimated 1,200 people were killed in the ethnic bloodshed and about 600,000 fled their homes.
His lawyers argued that justice could be met in his absence.
At least 67 people were killed when militants believed to be from the Somali al-Shabab group stormed the shopping centre last month.
In their ruling on Friday, judges were critical of the initial decision to give the deputy president a "blanket excusal" before the trial had even commenced.
He will be allowed to apply to miss portions of the case but his absence will only be allowed when it is absolutely necessary.
The BBC's Anna Holligan in The Hague says that the ruling reinforces the power of the ICC to hold the most powerful to account.
However, Friday's ruling could deepen tensions between the court and African leaders who accuse it of unfairly targeting their continent, correspondents say.
Mr Ruto has, so far, attended much of his case.
Friday's judgement could also affect the trial of Kenyan President Uhuru Kenyatta, whose trial on similar charges is scheduled to start next month.
The prosecution has decided to appeal against a decision to allow the president to miss parts of his trial.
Mr Kenyatta argued that attending the trial in The Hague would prevent him from governing the country.
Our correspondent says that Mr Ruto, Mr Kenyatta and journalist Joshua Sang comprise the "Big Three" accused of instigating and co-ordinating the post-election killings in which innocent people including women and children were shot and hacked to death.
But our correspondent says that was six years ago, and today Kenya is on the frontline of the battle against the global threat posed by al-Qaeda and its global affiliates.
Mr Ruto's trial began in September.
The deputy president is the first serving government official to stand trial in an international court.
Many experts in international law believe that his case reflects the apparently incompatible demands of historical restorative justice versus future global security.
Experts believe that the ICC is in an impossible position, fighting against the tide of international pressure to placate the government in Nairobi.