The trade body for Northern Ireland small businesses has called for changes to the way the government issues public contracts to private companies.
A report by the Federation of Small Businesses (FSB) has highlighted the problems small and medium-sized enterprises (SMEs) face in trying win public contracts.
The current public procurement system does not fully allow small businesses to realise their potential to sell to the public sector, the report said.
Problems which SMEs experienced included being paid late, the high costs of tendering a bid, lack of information on the possibility of future work, and the criteria of the awarding process.
Wilfred Mitchell, FSB NI policy chairman, said the report "sets out clearly the challenges our members face at all stages of the procurement process - from becoming aware of public procurement opportunities through to responding, receiving feedback on tender submissions and finally getting paid".
The report, produced in partnership with RCM McClure Watters, explained the public sector in Northern Ireland spends up to £3bn annually on buying goods, services and expertise.
Finance Minister Simon Hamilton, whose department oversees the public procurement system, said: "Public procurement makes a significant contribution to the local economy. Substantial progress in improving access to it for small and local businesses has been made. For example, in 2012/13 51% of contracts went to small businesses that have fewer than 50 employees.
"Almost 80% of our supplies and services contracts go to local businesses here in Northern Ireland.
For construction projects this is even higher, at 96%. These figures demonstrate how accessible public sector opportunities are in Northern Ireland," Mr Hamilton added.
Mr Mitchell drew particular attention to problems some businesses have faced in gaining payment for the work they have done.
"The FSB has particular concerns around some of the bad practice which has come to light in respect of late payment, including efforts by a Contracting Authority to disguise this," he said.
"Small businesses often do not have huge cash reserves to fall back upon, so prompt payment following the completion of contracts can sometimes make the difference between staying in business or not."