There remain doubts over whether Cardinal George Pell will return to Australia next month to give testimony to the Royal Commission into Institutional Responses to Child Sexual Abuse, after he failed to travel to the United States this week for a speaking engagement.

Australia's highest-profile Catholic, who is based in Rome where he manages the Vatican's finances, previously cancelled a planned trip to Melbourne in December to give evidence to the commission due to a "worsening" heart condition.

His testimony on how he responded to child abuse allegations as a priest in Ballarat and as the Archbishop of Melbourne was delayed until February this year, when a third public hearing into Catholic Church authorities in Ballarat is due to begin.

However, even church officials are unsure whether the 74-year-old will be able to return from Europe.