The institute, which recently unveiled, with training consultants TMI, the first ever full study into the UK's complaints culture, aims to improve customer service.
Stephen Connock, chairman and chief executive of the ICS, said: 'The rapid growth of the ICS has been remarkable.
'It confirms that organisations in both the public and private sectors are determined to recognise the professionalism and importance of their customer service staff, especially those operating at the front line.
Paul Coen, chief executive of Surrey CC, commented:
'We are delighted to be the 100th organisation to join the ICS. We believe passionately in the development and recognition of our staff. The Institute provides us with the opportunity to secure for them professional qualifications in customer service.
'We also look forward to working with companies and organisations in providing service leadership to the benefit of customers throughout the UK.'
Ian Smith, managing director of Oracle UK and Ireland and vice president of the ICS, added:
'Through the commitment and support of its organisational members, the ICS is making a significant contribution in raising the profile of the service industry throughout the UK. Like its members - including all those doing great work in key customer facing roles - the ICS is making a difference.
'Organisations are already seeing the benefits from their customer service staff whose confidence, competence and overall professionalism has increased through their association with the institute.'
The Institute of Customer Service is committed to developing people and systems to fundamentally raise service standards. It is backed by 100 leading UK companies and public sector organisations who share a common goal - to improve service standards by developing customer service staff and helping them to achieve recognition as professionals.
The ICS is particularly relevant to front-line people who are often the ambassadors, the public face, for their organisation. More than three million people in the UK work in customer service activities in the private sector alone. Many are part-time staff, women returning to work and graduates.
Individual membership is available to anyone working in customer service who holds either a N/SVQ in Customer Service or one of the Institute's own Awards in Customer Service.
The ICS has also been appointed by the DfEE as the National Training Organisation for Customer Service. As the NTO, the ICS works alongside other organisations across the UK to provide service leadership, promote lifelong learning and encourage the take-up of customer service qualifications, particularly by those in front-line service delivery roles.