With icy weather gripping the UK and snow forecast for the weekend, Dame Sally Davies warned that vunerable people face a higher risk of illness.

She urged people to look out for those at risk as the mercury plummets. Temperatures in parts of England fell to -11C on Friday morning.

'During a cold spell, older people and those with underlying health problems, such as heart disease or respiratory conditions, can all be at higher risk of becoming ill,' she said.

'Everyone should look after themselves this week and also keep an eye out for those around you.'

Dame Sally added that older people should take extra care to avoid slips and falls in the icy weather.

'If you do not want to brave the cold outside there are simple ways to keep warm at home, such as drawing curtains to block out draughts, drinking plenty of hot drinks, and wearing lots of light layers of clothing.'

The HPA said there was a 100% probability of severe cold weather, icy conditions and/or heavy snow between 10am on Friday and 10am on Tuesday in parts of England.

'This weather could increase the health risks to vulnerable patients and disrupt the delivery of services. Please refer to the national Cold Weather Plan and your trust's emergency plan for appropriate preventive action,' the HPA said.

Despite the recent cold weather, levels of flu have not risen above baseline rates so far this winter.

• Visit our Interactive Guide to the Flu Season 2011/12 to see the latest flu figures