The legal profession has received a sharp warning from the Court of Appeal on the dangers of giving lay clients free rein to put forward unsubstantiated allegations in court. The warning came in the case of Thatcher v Douglas & anor and is one that Donald Ramsbottom, a partner in Hayling Island firm Michael Daltons, says counsel and solicitors should heed.

The warning was issued during judgment on what seemed a routine row over rights to use a concrete slipway in the Solent area. In the county court at Portsmouth in 1993 Mr Recorder Donne QC found for the plaintiff, represented by Ramsbottom. The Court of Appeal backed that decision.

But shortly before the appeal was due to be heard the defendant launched an attack on the integrity of Donne, accusing the judge of corruption and bias. It was this which appeal judge Lord Justice Nourse said made an "otherwise unremarkable" case "unique". And he rounded on the defendant appellant's counsel Graham Lyons for advancing this aspect of his client's case.

He said that while the court appreciated counsel was put in a difficult situation after receiving instructions to make the claims, Lyons made a "grave error of judgment" in doing so.

It was, said the judge, sitting with Lords Justices Hutchison and Thorpe, the duty of counsel to put a client's case. But that duty could not extend to advancing the client's unsubstantiated belief that the judge was corrupt or biased. He said that lawyers faced with such a situation should decline to comply with the instructions or withdraw from the case.

Ramsbottom says the case has to be seen as a warning to both sides of the profession on the dangers of putting forward a client's claims when they are not backed by evidence. "The message is very clear – do not always do what your client says, don't carry on with outrageous instructions," he said.

In such a situation, Ramsbottom says it is important to check a legally aided client remains covered by aid if claims are advanced. In this case he says he queried this beforehand but the claims were continued.

The Court of Appeal also raised the point and stressed that specific authority should have been obtained from the Legal Aid Board for the allegations against the judge to have been introduced.

The issue is one which any lawyer facing such allegations should raise, says Ramsbottom, if only as a way to legitimately tell clients their instructions to advance unsubstantiated claims cannot be followed.

"Some lay clients can be difficult and very insistent but it is open to the profession when dealing with such clients to have such arguments effectively barred on the basis of refusal of legal aid backing to make them," said Ramsbottom.

On another footing the case also emphasised the prohibitive legal costs that can mount in relatively low key cases.

Although described by the court as "unremarkable", the slipway battle has run up a £100,000 bill for the non-legally aided plaintiff said Ramsbottom. The plaintiff died a short time before the appeal hearing, but the costs bill has virtually bankrupted his estate.

"It's just another example of the fact that you either have to be extremely rich or extremely poor to get involved in litigation and that there is no in between," said Ramsbottom.