Withers and Speechly Bircham are to vote next week on a potential merger which could create a UK top 25 law firm known as Withers Speechly Bircham (WSB).

Speechly's interim vote is on Tuesday (21 May) and Withers' is on Wednesday (22 May), with the news coming after the private client focused firms issued a joint statement on 25 March confirming preliminary discussions were taking place.

A combined firm would have revenues of around £170m, after Withers reported turnover of £113.3m for 2011-12, with the equivalent figure at Speechly coming in at £57.6m. Withers has profits per equity partner (PEP) of £386,000, compared with £299,000 for Speechly. It is currently unclear whether the firms will fully financially integrate if a merger is confirmed.

A combined firm would have around 520 lawyers, including roughly 200 partners, based on Withers' 2011 calendar-year financial results and Speechly's figures for 2011-12. It is understood that some partners could leave as a result of a tie-up, although exact numbers are not yet known.

The proposed new firm will be called Withers Speechly Bircham (WSB) in Europe and Withers Bergman in the US, as a result of Withers's 2002 merger with New York firm Bergman Horowitz & Reynolds.

According to one ex-partner, some members of Withers' litigation team have reservations about a tie-up. "The previous vote for the merger in Italy last year only just squeezed through. Withers' management have an uphill task in persuading partners to go ahead. Withers has had a good year and seen a shallow climb whereas Speechly has not done so well. The commercial side of Withers will be in favour but there is concern that the firm has acted for individuals."

The firm has been piloting an open-plan format in its London office at 16 Old Bailey, taking out office partitioning to make space for Speechly to move in.

The votes follow a number of senior level exits from Withers, with European corporate head Adam Duthie leaving the firm last month, while in October last year Astrid Owen left to join US firm McDermott Will & Emery in London.

In a statement, Speechly said: "We are happy to confirm that our discussions are continuing. We can also confirm that, in due course, there will be an interim vote as part of a planned timetable of discussions. The vote will be an opportunity to discuss how negotiations are progressing and whether to move them into a more detailed phase."

In addition to its London headquarters, Withers has bases in Geneva, Zurich, the British Virgin Islands, Hong Kong, Singapore, as well as three US offices in New York, New Haven, and Greenwich, while Speechly has offices in London, Luxembourg and Zurich.