The above is a fairly well known rule imparted to barristers during their training at an early stage. The rule was recently considered in relation to an expert witness, although it also applies to lay witnesses. In my view the rule merely accords with fairness and common sense.
The case in question was Griffiths v TUI Ltd [2023] UKSC 48. In this case an expert's evidence was rejected; the Defendant's counsel sought to impugn the expert during submissions without having cross examined them.
The rule was upheld as a general principle and the Claimant's appeal was upheld, as it was decided that the Court was wrong to reject the expert's evidence. Some exceptions to the rule were recognised, but none were deemed to apply. I would suggest that if there is any doubt, counsel should cross examine.
The exceptions included:
If the challenge is secondary to the main issues so that fairness does not require the witness to have the opportunity to reply.
The evidence is manifestly incredible.
There is a bold assertion of opinion in the expert's report.
There is an obvious mistake.
The evidence is based on incorrect factual assumption