Irwin Mitchell has confirmed a significant fall in profits in the wake of its merger with Thomas Eggar, with profit before tax plunging 25.7% in 2015-16.

The results, which include some revenue from its recent merger partner for the first time, show revenue hitting £221m compared with a restated figure of £204.5m for 2014-15.

Profit, however, fell to £12.4m in 2015-16 from £16.7m in 2014-15, with the firm blaming this on the December merger and subsequent integration efforts.

In 2014-15, Irwin Mitchell announced revenue of £210.6m, restating it this year due to a change in accounting standards. Thomas Eggar posted 2014-15 revenue of £41m, however a spokesperson for Irwin Mitchell said only revenue brought in by the smaller firm since December was included in this year's results. In its next full year of results, the firm expects to reach a revenue of around £250m.

Andrew Tucker, Irwin Mitchell's chief executive, said: "There are many reasons for real confidence in our business, despite the reduction in profit this year. That is a short-term issue driven by the significant investment in the merger to ensure it was a success."

He added: "The board is comfortable that sacrificing profit in the short term will deliver greater benefits to the business in the medium term, as we reap the return on investment and the improved strength and breadth of depth the merger has given us."

Irwin Mitchell merged with southeastern firm Thomas Eggar in December 2015, giving it 67 additional partners and new offices in Newbury, Southampton, Chichester, Worthing, London and Gatwick.

Part of the rationale was to target the private wealth sector. In April 2016, it brought together the private wealth teams from Irwin Mitchell, Thomas Eggar and Berkeley Law, a private client boutique acquired by Irwin Mitchell in 2014, under the single umbrella of Irwin Mitchell Private Wealth.

Irwin Mitchell has seen a number of resignations in recent months from its business legal services group. Eight partners have resigned from its London real estate team, with five going to Dentons, two to Osborne Clarke and one to Howard Kennedys.

A further three partners left in March – London head of employment Chris Tutton, and litigators Simon Ellis and Daniel Brumpton.

It has however brought partners in during the period, including corporate partner Philip Goldsborough from Pinsent Masons in Leeds and pensions partner Andrew Ashley Taylor from DWF in Manchester.