RMJM employs 850 people but staff have been complaining of late wages following cash-flow problems.
The Morrison family has now stepped-in to help led by RMJM chief executive Peter Morrison and chairman Sir Fraser Morrison.
Sir Fraser built up the Morrison Construction firm which was sold in 2000 for £262m.
The extra funding for RMJM follows two years of severe trading problems after the practice was hit by the economic downturn and late payments by clients in the Middle East and Russia.
Last year, the firm took on former Royal Bank of Scotland chief executive Sir Fred Goodwin as an adviser on international business.
The Morrison family is to inject £5m in share capital, with a further £3m as a loan.
Peter Morrison said: “The economic downturn has meant a very difficult couple of years for the profession but I’m immensely grateful to our teams and our partners worldwide for their patience and the exceptional quality of work they have continued to produce.
“Trading conditions continue to be challenging for the architecture sector but we are now seeing definite signs of recovery.
“The Far East continues to trade well and we are pursuing many significant new opportunities there.
“After two very difficult years in the Middle East, the economy there is getting back on its feet and our US business is also starting to benefit from the recommencement of a number of projects which were put on hold during the height of the recession.”
News of the funding deal comes just days after it emerged there had been an exodus of senior management at the company’s Hong Kong office amid a row over non-payment of salaries – while 33 other staff in the China team have been poached by rival firm Aedas.