The Government's chief adviser on the human form of mad cow disease today warned of a possible new wave of the illness which could kill as many as 350 people in the UK.

Professor Chris Higgins highlighted a case being investigated by scientists in which a patient dying from variant Creutzfeldt-Jakob Disease (vCJD) appears to have a different gene type from previous British victims.

Prof Higgins, who chairs the Government's Spongiform Encephalopathy Advisory Committee, said that if confirmed, this case could mark the start of a second wave of deaths over the coming years involving anything between 50 and 350 people.

The first wave of vCJD, caused by eating infected beef products in the 1980s and early 1990s, has been responsible for 164 deaths, peaking at 28 in 2000 and dwindling over the following years to about one case annually.

All of these victims belonged to a gene type known as MM. Every individual inherits either an M or V gene from each of their parents, determining their vulnerability to vCJD. Those with two M genes - the MM group - make up 42 per cent of the population and are thought to be more susceptible, while those with one of each - the MVs - are thought to be more resistant and constitute 47 per cent of Britons.

Clinical tests suggest that the new patient has an MV gene type, although this cannot be definitively confirmed until brain biopsy is carried out after death.

The possibility of an MV sufferer has raised concerns among scientists that the illness may have a longer incubation period among this group than among MMs and that other victims may soon start to show symptoms of an infection contracted as long as two decades ago.

Prof Higgins told BBC2's Newsnight: "This case has not yet been confirmed, but assuming it is, we would estimate that the number of MV cases at a maximum would be about 300 to 350.

"That, I have to emphasise, is a maximum estimate, because all the studies in animals suggest that MV gene types are likely to be far less infective than the MM gene type where we have seen the cases up to now.

"So it is probably between 50 and maybe 350 new cases."

Newsnight quoted an article due to appear soon in the medical journal The Lancet, in which two vCJD experts warn: "A second wave of vCJD with a longer incubation time might hit these shores, but we don't know if this will be a tidal wave or an imperceptible ripple."

The mother of Andy Black, from Southsea, Hampshire, who died from vCJD a year ago this week at the age of 24, will today go to 10 Downing Street to raise awareness of the concerns.

Christine Lord's son was found to have the MM gene type, but she is concerned that the public is unaware of the danger of a new wave of cases.

She told Newsnight: "This young man is dying of vCJD and he is an MV and I think we need transparency about this. What will cause more panic is people trying to hide things and keep things from the public.

"I want people out there to know that vCJD hasn't gone away and it is still killing people and now it looks like it is a second wave."

Ms Lord will today present a petition addressed to Prime Minister Gordon Brown, raising questions over how the BSE crisis was handled by the former Conservative government and asking whether the country faces a further wave of vCJD cases.

She will be accompanied by Liberal Democrat Portsmouth South MP Mike Hancock.

A spokesman said: "Her research since has raised serious questions about the Conservative government's handling of the BSE crisis, and has highlighted the need for action to prevent further deaths from infected blood donors.

"Christine will be asking the Prime Minister if we are seeing the beginning of a new wave of vCJD with the first confirmed victims of the more common genotype and another estimated 300-plus deaths."