Stephenson Harwood is facing a counterclaim by the trustees associated with British property mogul Robert Tchenguiz, whom the firm is chasing for more than £6 million in unpaid legal fees.

The firm last year lodged a High Court claim against Tchenguiz's trustees for unpaid bills under a retainer, including for its work on an ultimately unsuccessful 2015 court case involving the trusts' former business partner, Abu Dhabi investment company Aabar.

The three trustee companies – the Geneva Trust Company (formerly Rawlinson & Hunter Trustees), Fort Trustees, and Balchan Management – have now lodged a counterclaim against Stephenson Harwood, seeking damages for wrongful termination.

According to court documents, in 2018 the firm ended its representation of the trusts in various ongoing court proceedings before they had concluded, which the firm argues was due to their failure to pay outstanding invoices that – according to the particulars of the claim – total about £6 million including interest.

According to the defence and counterclaim document, the trustees allege that Stephenson Harwood's termination of its retainers left them needing to find "alternative representation at short notice", resulting in "loss and damage".

The trustees argued that the firm agreed it would seek payment only once "funds became available" – an arrangement discussed between Tchenguiz and Stephenson Harwood's chief executive Sharon White in March 2018.

A spokesman for Stephenson Harwood said: "The debtors have responded, as can happen in fee-recovery cases, by seeking to raise alleged issues about the services provided to them or other reasons why they should not pay the sums due.

"The counterclaim is not, as Mr Tchenguiz's representatives apparently wrongly allege (and so is wrongly reported by the Mail on Sunday), in relation to the conduct of the Aabar trial. We will be addressing this with both of them. The matter being subject to court proceedings, the firm has no other comments than that it will pursue its claims to judgement."

The Geneva Trust Company and Fort Trustees did not respond to requests for comment. Balchan Management could not be reached for comment.