Firms with a turnover of between £10 million and £100 million represent less than 1% of businesses but generate 22% of economic revenue and 16% of all jobs.
The CBI report called Future Champions: unlocking growth in the UK's medium-sized businesses, uses new analysis conducted with the help of McKinsey & Company.
The CBI is calling for a broader range of finance to be made available to medium-sized businesses (MSBs). With banks' lending constrained, these firms can no longer rely solely on bank lending for long-term growth capital to invest in their companies.
Business owners and managers will be crucial to unlocking MSBs' true potential, and the CBI makes a series of recommendations for building their capabilities and inspiring them to strive for even greater growth.
John Cridland, CBI director-general, said: "Medium-sized businesses are truly a forgotten army, and now is the time to unlock their potential.
"We should be championing, nurturing and encouraging our medium-sized firms so that more of them grow and create jobs. For too long these companies, which could inject tens of billions of pounds into our economy, have fallen under the radar of policymakers.
"I want the UK to have its own version of the German 'Mittelstand' - a backbone of medium-sized firms which export, innovate and generate growth. These future champions would help the UK weather unexpected economic shocks, and act as a new engine for growth.
"To achieve extra growth, medium-sized firms must have access to new kinds of finance. This means opening UK bond markets to medium-sized businesses, encouraging use of venture capital, and making it easier for large companies to invest in medium ones, possibly in their supply chains."
The contribution MSBs make to the UK economy is smaller than in France and Germany, where they contribute a greater share of total revenue and generate a significantly higher proportion of jobs. The main reason for this is that, while some MSBs perform extremely well - the "gazelles" - others are lagging behind.
But many MSBs operate in sectors with high productivity growth. And if MSBs had been able to raise their productivity between 2002 and 2007 as much as large firms operating in the same sectors, their annual rate of productivity growth would have been one per cent higher than the largefirms.
Figures provided to the CBI by NESTA reveal that just 6% of MSBs create 60% of all the new jobs created by the sector. If more firms could reach their potential to grow, this could help achieve an extra £20bn to £50bn by 2020, a major boost to the UK's GDP.
MSBs could play a crucial role in rebalancing the economy, and create new jobs in areas most affected by public spending cuts. In the North East, where unemployment reached 11.3% in the three months to August 2011, MSBs already account for about 20% of all jobs. Mid-sized firms also represent 30% of the UK's manufacturing base.
MSBs have traditionally lacked channels of dialogue with Government. Just one member of the prime minister's Business Advisory Council is currently an MSB, and Government policy usually focuses on the very large firms or the very smallest.
The CBI is urging the Government to recruit more MSBs to the Prime Minister's Business Advisory Council, and to ensure its policy initiatives cater for MSBs.
