The Foreign & Commonwealth Office (FCO) has been forced to open up its panel to major UK law firms as it seeks expert legal advice on negotiating trade deals in the wake of the Brexit vote, The Lawyer can reveal.

The decision will see its legal spend skyrocket, with leading firms including Clifford Chance and Herbert Smith Freehills understood to be pitching for the work.

Its Public International Law (PIL) panel has previously been restricted to barrister appointments only. However, the FCO has extended the invitation to pitch for places to some of the UK’s biggest law firms.

Firms have until the end of October to submit applications.

An insider said the FCO has informed firms the PIL panel will be subsequently used by the Department for International Trade and the Department for Exiting the EU – also known as the Whitehall Brexit unit – in their own trade negotiations as Britain prepares to leave the union.

The Brexit department has already run up legal bills of £270,000 solely on internal legal counsel, it emerged last month. Minister David Jones told Parliament that so far “no spend has been incurred in relation to external firms”. That sum is expected to dramatically shoot up when it begins using PIL panel law firms.

According to the tender documents, the PIL panel “advise the Government on matters of public international law and international trade law”. The panel is split into three tiers, with the top level appointments awarded to firms and barristers with “in excess of 10 years practice experience” in order to “deal with the most complex public international law matters”.

The FCO and attorney general’s office are turning to commercial law firms for advice on a complex set of deals to establish the UK’s place in the world after Brexit, including hashing our free-trade agreements in Europe and internationally, and a membership deal with the World Trade Organisation.

By opening up its panel, the FCO has offered UK law firms a crucial seat at the table when major decisions on international relationships after Brexit are made.

However costs associated with public bodies drafting in the top private firms for legal advice have already caused concern.

Herbert Smith Freehills recently hit headlines for its work representing part publicly-owned bank RBS on its £4bn shareholder dispute over its 2008 rights issue. The firm has already run up a bill of more than £100m and that figure is expected to rise.

Clifford Chance also found itself at the heart of a row in February related to law firm charge-out fees. The Centre for Policy Studies (CPS) found the magic circle firm was among a group of City lawyers charging clients more than £1,000 an hour.