The Government is being urged to help medium-sized firms grow by reviewing the tax system for entrepreneurs and improving access to skilled workers.
The CBI said medium-sized companies had a key role to play in boosting economic growth.
In her first keynote speech as director general of the business group, Carolyn Fairbairn will tell a London conference that mid-sized firms needed more support.
"As a country, we're good at telling the beginning of the story. The traditional story of "start-up Britain", a single person, with a single idea and a single-minded determination.
"We're also good at telling the end of the story. The story of 'Brand Britain' of big, well-known firms which people recognise and interact with every single day.
"Yet the 'missing link' is the most interesting part. It's the part where entrepreneurs grow their great idea. Where they increase their profit, hire more people and expand beyond national borders," she will say.
Antonio Horta-Osorio, chief executive of Lloyds Banking Group, which is supporting the conference, said: "Medium-sized businesses are playing a critical role in the UK economy, creating jobs and growth, and we are determined to help their continued success."