The EU has told UK military staff that they will not have their secondments to Brussels renewed after Brexit, in the latest setback to the increasingly fraught negotiations over defence and security collaboration after Britain leaves the bloc.

While both sides say they are seeking an “ambitious” agreement on security and defence co-operation, talks have proved much harder than expected, with only limited progress made to date. UK and EU officials are discussing foreign policy co-operation in talks this week.

The dispute over the secondments again underlines the stand-off between Brussels and London over elements of a future security and defence agreement.

Of the 14 British personnel seconded to Brussels, 13 of them are serving in the EU military staff while one is working in the crisis management and planning directorate of the European External Action Service.

UK officials argue that the positions, which replicate similar arrangements Britain has with US defence forces and command structures, enables smoother co-ordination on joint military missions and gives British commanders early sight of any key EU operational decisions. Britain has around 200 military personnel embedded in, or on secondment in the US, and there are also about 135 exchange officers.

The Ministry of Defence said British secondments are now being considered by the European Commission on a case-by-case basis to ensure there are no conflicts of interest in potentially sensitive areas.

Sir Stuart Peach, chief of the UK defence staff, urged negotiators to find some common ground © Mark Rawlings/PA

The department added that there are “challenges surrounding new applications and extensions” of secondments although it said it was confident there are no present restrictions to the UK’s ability to second officials during the implementation period after Britain leaves the bloc next March.

A senior EU official commented recently that the UK has “a lot of appetite for the secondment UK officials” into EU bodies “which would require the EU to change its rules again” on access for so-called third countries (non member states).

Michel Barnier, the EU’s chief negotiator, has compared some UK requests on internal security co-operation to a country “at the beginning of an accession process” to join the EU.

The commission is also adopting an uncompromising approach on European Defence Fund, which would limit the depth of involvement for UK companies.

On Tuesday, Air Chief Marshal Sir Stuart Peach, the chief of the UK defence staff, urged negotiators to find some common ground. “At times like this the military to military relationships matter more than ever,” said Sir Stuart, following a speech to UK think-tank Policy Exchange.

“The history and geography of our nation doesn’t change because of Brexit. We have to find ways of working together with our European friends and partners. We have to find a way to keep the door open as structures evolve for third party access.”