The Surrey-based practice, which was the eighth-largest firm in the UK according to last year's AJ 100, offers services in everything from architecture to town planning, but saw its profits slashed by half in 2006 compared to its 2005 year-end results.
Although the practice saw a slight rise of 2.9 per cent in turnover in the last year, it was not enough to prevent the drop in profits.
Despite this, the company's directors are expecting a further growth in turnover for 2007.
PRP chairman Barry Munday said: 'Those figures don't really mean very much to be honest.
'They are just what Companies House has reported. We are a private limited company with 50 shareholders so we make the decisions on how we distribute our profit - not external shareholders. We are not a PLC which the city is looking to invest in.'
PRP has just revealed plans for a vast new scheme near Cardington Airfield, where it plans to build more than 400 homes close to two Grade II-listed airship sheds.