An outbreak of swine flu could prove catastrophic for some firms – and employees who fall ill should stay at home, business leaders warned today.

Martin Traynor, chief executive of Leicestershire Chamber of Commerce, spoke out after health officials said there were hundreds of cases in the city and county and the spread of the infection could no longer be contained.

The Government yesterday warned that by August there would be more than 100,000 cases a day.

Mr Traynor said: "An outbreak of swine flu could prove catastrophic for some businesses, particularly in the current economic climate."

Smaller firms would be particularly affected along with the county's tourism industry, he added.

He said: "Most firms will have a disaster recovery plan but whatever industry you are in – manufacturing or tourism – if you lose 50% of your workforce your input will drop. With tourism it could mean that people don't come to the county and it will have a serious effect on business.

"The advice to employees is to stay at home if they are unwell so any spread of infection can be contained as much as possible."

Although there are 147 confirmed cases in the East Midlands, it is estimated hundreds more have either had or are recovering from swine flu.

Yesterday, workers at Leicester City Council were told a colleague at the New Walk offices had now been confirmed as having the virus. Staff are being urged to report for work as usual "whilst remaining aware of the symptoms."

In an e-mail to staff Alistair Reid, business continuity planning director, said: "It is important that we continue to provide our services."

The 1,500-strong workforce at the Alliance and Leicester headquarters in Carlton Park has also been e-mailed to say a colleague has the infection.

At least five schools in the city and county are believed to have pupils suffering from swine flu.

Doctors have stressed that just because the virus cannot be contained does not mean cases of swine flu are becoming more serious. They have urged people with concerns to contact NHS Direct and read as much as they can about the illness.

Ivan Browne, a public health consultant with NHS Leicester City, said: "We are expecting an escalation in numbers over the next three or four weeks but this doesn't mean the virus is becoming more dangerous.

"Some may even be happy managing the illness at home." Dr Simon Mardel, an accident and emergency consultant at Leicester Royal Infirmary, said: "A lot of people are frightened because it is a new virus. But the more they know about it the more their fears will be allayed."

He added: "We are always happy to see people if their symptoms are causing undue concern.

"However, there are really good detailed plans in place for people to check their symptoms and see if they need to be here. Unless they follow them they will not work.

"In New York and Chicago some emergency departments have seen double the number of attendances. If that were to happen here it would be unsafe for patients and staff."

The Government announced that from now on, Britain is in a "treatment" phase for the virus. This means there will be no more swabs taken from patients to confirm suspected cases. Family and friends of people with swine flu will no longer get anti-flu drugs. Instead, people who think they may have the virus are being asked to call their GP or NHS Direct.

They will be told to stay at home then get a unique reference number to give to a friend or relative who can pick up the medication.

Mr Browne added: "It is important for people to identify a 'flu friend' who can collect any medication needed. It is also really important people have a thermometer and know how to take their own and their children's temperatures."

Health Secretary Andy Burnham told the House of Commons yesterday. "We have reached the next step in our management of the disease.

"Our national focus will be on treating the increasing numbers affected by swine flu.

"We will move to this treatment phase across the UK with immediate effect."

There are now 7,447 laboratory-confirmed cases in the UK after a "considerable rise" last week. In the UK three people – who all had underlying health problems – have died after contracting the virus.

Mr Burnham said: "Cases are doubling every week and on this trend we could see over 100,000 cases per day by the end of August."

The Government has signed contracts to secure enough vaccine for the whole population, he told MPs.