The former Labour leader's daughter Margaret is married to Peter Jay, who worked for the brothers' late father, Robert Maxwell, as chief of staff in the mid-1980s.
"It's certainly true to say that Kevin and Ian are confident that they can put into place several eminent witnesses who might appear on their behalf during the trial,'' said one source close to the brothers.
Lord Callaghan, who declined to comment on the matter this weekend, was prime minister between 1976 and 1979. He continued as Leader of the Opposition until 1980.
Last week there was a public outcry when it emerged that the legal aid bill for the Maxwell brothers' defence had surpassed £4m. Legal experts say that the bill could easily exceed £10m if a trial goes ahead.
The brothers and four other former executives face fraud charges arising from their roles in the publishing empire created by Robert Maxwell, who died in November 1991.
Following an application, a pre-trial hearing is scheduled for 13 February On Friday afternoon the Independent on Sunday was served with a court order that prevents the newspaper reporting the nature or content of this hearing until it has been resolved.