Berwin Leighton Paisner (BLP) could push through its merger with Bryan Cave as soon as 1 January next year, according to sources close to the firm.

It is understood that BLP partners are expecting to vote on the deal in the next four weeks.

One BLP partner said: "It's very complicated, but I would say that this is the aspirational timeline."

BLP and Bryan Cave confirmed the merger discussions earlier this month (16 October) in a joint announcement. It is understood that the pair are aiming for close financial integration, as referenced in their statement, in which Bryan Cave chair Therese Pritchard said the combined firm "would be one of only a handful of global firms operating in a one-firm structure".

Fully integrated transatlantic mergers are rare, and present significant challenges due to the different tax and accounting systems traditionally used by UK and US firms. US firms typically rely on cash accounting, where income must be received by a calendar year end of 31 December, while UK firms normally use an accrual system, counting all billings recorded during the financial year as revenue, regardless of whether clients have paid.

If the merger goes ahead, it will create a 1,500-lawyer firm with combined revenue of about £730m. The merged firm would have 32 offices in 12 countries around the world.

Any deal is subject to the resolution of conflict issues and approval by partners at both firms.

Rumours of BLP's ongoing desire for a merger have continued to circle the firm since its talks with US firm Greenberg Traurig ended last year, with the firm this summer denying suggestions that it had held talks with Norton Rose Fulbright.

Meanwhile, Bryan Cave has been on its own quest for growth, coming close to acquiring Dickstein Shapiro in late 2015, before a deal collapsed shortly after Christmas.

BLP declined to comment.