Britain is set for a heatwave which could see temperatures soar to 35C but a health warning has been issued amid concerns that lives could be at risk.

Vulnerable groups including the elderly, young children and people with breathing difficulties have been urged to stay cool as the hot weather pushes across the UK from Europe.

With temperatures on Wednesday forecast to reach 35C (95F), D r Angie Bone at Public Health England said there could be more deaths than usual.

She said it seemed "likely" that the Met Office would issue a level 3 heatwave alert - which requires community support for at-risk groups, media alerts about keeping cool and a review of safety at public events. She added: "It is possible that we will see an excess mortality but it is too early to tell.

"We know that high temperatures do have an impact on health, particularly on older people and young children and people with chronic diseases."

Dr Bone said she hoped the action would " minimise the impacts that we will see on health and wellbeing and health services".

The director of policy at Independent Age - a charity that helps older people live independent lives - said he hoped the move would stop heat-related deaths.

Simon Bottery said that when similar actions were taken in 2009, there were 300 excess deaths.

"What we are hoping is that we get even better at it this time round."

The mercury could reach 35C (95F) on Wednesday - hotter than Rio de Janeiro - and temperatures are predicted to stay high all week.

Social workers and other council staff have been placed on high alert to help those who might be struggling.

Izzi Seccombe of the Local Government Association said: "The hundreds of deaths caused by extreme heat each year are avoidable. Councils are determined to reduce the toll as much as possible but they cannot do it alone.

"Local people can make a massive difference by helping us identify other residents who might need some advice or practical help."

The heatwave is being caused by a warm front and tropical continental air mass from Europe pushing across the country, bringing high temperatures and humidity.

However, severe thunderstorms are predicted to strike western and northern parts of Britain by Wednesday afternoon, and temperatures will dip on Thursday - but are likely to rise again before the weekend.

Dr Paul Cosford, director for health protection at PHE, said the high temperatures could be dangerous for vulnerable groups.

He said: "Older people and those with long-term illnesses are particularly vulnerable to the effects of very hot weather, so it's important to look out for them - help them stay hydrated with plenty of cool drinks, and keep indoor areas as cool as possible."

The medical director of West Midlands Ambulance Service, Dr Andrew Carson, warned that those with respiratory illnesses should stay indoors.

He said: "A prolonged heatwave poses a real danger to those patients with emphysema, asthma and long-term breathing problems.

"The hot weather makes breathing difficult, even for healthy individuals. That's why we suggest people with respiratory illnesses should remain indoors as much as possible."

The Met Office issued a level 2 heat-health alert, warning that there is a 60% or greater chance of temperatures being high enough to affect health. The Met Office said the mercury could rise to 33C (91F) in London on Wednesday, and reach the high 20s in Scotland and northern England.

Paul Knightley, the forecast manager at MeteoGroup, said the tropical continental air mass moving from North Africa, Spain and Portugal to Britain could make Wednesday the hottest day for several years.

He also warned that the sun was "about as strong as it can be" and UV levels will be quite high.

He added: "One issue will be the temperatures won't be dropping as low as perhaps we might normally expect overnight. Overnight temperatures in many areas might be 16C-19C (60F-66F), so the cumulative effect from day and night will actually be really hot."

Heatwaves have been deadly in recent years. An Office for National Statistics report found that the heatwave across Europe in August 2003 caused 2,091 excess deaths in England, particularly affecting the over-75s in London.