Joanne Fraill, a juror in a trial in Manchester, was sent to prison for contacting an acquitted defendant on Facebook while the cases about the co-accused were ongoing.

Other incidents involved two jurors who went online to research a case, an allegation of bullying in the jury room, and jurors who spoke to a defendant in a pub next to the court.

Lord Judge has now expressed concern about the impact of communications technology, and used his foreword to the Court of Appeal Criminal Division’s Annual Review of the Legal Year to warn of the need to preserve the integrity of the jury system.

He wrote: “I remain concerned at the ease with which a member of the jury can, by disobeying the judge’s instructions, discover material which purports to contain accurate information relevant to an individual case or an individual defendant.

“I am also concerned that the use of technology enables those who are not members of the jury to communicate, in both directions. In the context of current technology, we must be astute to preserve the integrity of jury trial and the jury system.”

A 2010 study of juries, commissioned by the Ministry of Justice, found that 13 per cent of jurors admitted to seeing information about the case on the internet. This figure rose to 26 per cent in high-profile cases.

The Court of Appeal received nearly 7,000 applications last year, according to the annual report, a slight fall on the previous year. More than one in five was against conviction, and the rest were against the length of sentence.