Mr Bannon appeared separately on Tuesday before a Congressional panel. Congress is holding its own inquiry into the allegations.

However the summons could be a negotiating tactic by Mr Mueller to persuade Mr Bannon to agree to be questioned by investigators in the less formal setting of the special counsel's offices in Washington, the paper's source added.

As prosecutors hunt for criminal misdeeds, they have a number of arrows in their quiver. One of the more powerful is the grand jury subpoena.

Putting witnesses in the glare of the courtroom spotlight, in front of a panel of jurors, with every word noted and every answer compelled, can be quite revelatory.