Dundas & Wilson energy and infrastructure head Keith Armstrong has resigned from the firm after "irregularities" were discovered in a tender pitch.

According to a report in Scotland on Sunday, Armstrong obtained confidential information about the client in question, thereby prejudicing the tender. Dundas withdrew the bid when the issue came to light and is now conducting an internal investigation, after referring the matter to the Law Society of Scotland.

Interim joint managing partner Caryn Penley said in a statement: "It was brought to the attention of the board that there were irregularities in a tender process in which the firm recently participated. As soon as we ascertained this we immediately withdrew our tender to maintain the integrity of the process."

"The circumstances of this are profoundly disappointing for all concerned. In the interests of everyone involved we have acted on this as quickly as possible. Once our investigations are finalised the matter will be referred to our board for further action and pending that, it would not be appropriate to make further comment."

Armstrong became sole head of energy and infrastructure after co-chief Andy Renton left the top 50 firm in October last year. Deals he has worked on include Aquila's sake of its interests in Midlands Electricity to Scottish and Southern Energy and Morrison Construction's joint venture with Edison Capital.

The news comes with Dundas set to elect a new managing partner and chairman this week. Candidates put their names forward on 8 June, with elections to be held at a partners meeting in Scotland this Wednesday (20 June). Interim managing partners Penley and Allan Wernham – who took over when predecessor Donald Shaw stepped down unexpectedly in March – are standing on a joint ticket. Corporate partners Colin Massie and Michael Polson are also standing as separate candidates.

Chairman David Hardie, who will oversee the election process, has also announced his intention to resign after it is completed, after carrying out three years of a four-year term. Real estate partner Iain Lindsay and intellectual property partner Laurence Ward have both nominated themselves to replace Hardie, with the new chairman expected to be in place by August.

The results of the elections will be announced the same day, with the new managing partner or partners set to formally take up leadership of the firm as of 1 August.