Two UK accountancy firms are in advanced talks over a merger deal that will create a business with around 3500 staff and more than 30 offices.

BDO and PKF confirmed plans for a tie-up of their operations, bringing together the UK's sixth biggest and 12th largest accountancy firms with combined revenues of nearly £400 million.

The groups said it was too early to say what the impact would be on staff and offices, but stressed they had a "commitment to growth".

The deal – for an undisclosed sum – is expected to take place early next year and will be the first merger of its kind in the market for 15 years, according to the groups.

Simon Michaels, managing partner of BDO, said: "The merged firm will have a strong balance sheet, as well as unrivalled sector and geographic strength in many areas."

The merger would see PKF, which has more than 1200 employees and 20 UK offices, become part of BDO International after the merger and be rebranded as BDO.

PKF, currently handling the administration of Portsmouth Football Club, has seen revenues come under pressure over the past year, with total income down by £5 million to £103.3m in the 12 months to March 31.

Part of PKF International, its UK operation is headquartered in London and has major regional offices including sites in Birmingham, Manchester, Edinburgh and Glasgow.

BDO has been appointed to wind up oldco Rangers. The firm, which has 13 UK offices including one in Glasgow's York Street, had UK fee income of £281.5m in the year to last September 30. It is the UK's sixth biggest, after PwC, Deloitte, KPMG, Ernst & Young and Grant Thornton.