Boris Johnson’s plan to seal a deal with Brussels on the future relationship with the UK by the end of December has been described as “fantasy land” by EU officials, as a leaked letter revealed the scale of the bloc’s inability to function during the coronavirus pandemic.

The European council headquarters, where member states’ positions are coordinated, is only able to hold one daily video conference due to a lack of facilities. The capacity to carry out work is 25% of what it would usually be.

The leaked letter from Michael Clauss, the German ambassador to the EU, to his political bosses in Berlin said a range of issues not key to rebuilding from the pandemic would need to be sidelined as a result.

An EU official said while the European commission, which is in charge of talks with the UK, had better facilities for remote negotiation, it would be impossible for the member states to have the same input through the EU council. This would make a successful negotiation nearly impossible.

The December timeline for agreeing a deal with the EU “which was already hopelessly optimistic” was described as “like fantasy land”, by one source. The UK can agree with the EU to extend the transition period by up to one or two years, if necessary, but Downing Street insisted it would not do so.

The planned schedule of negotiating rounds on the future relationship was abandoned after the first week due to the pandemic. The UK’s chief negotiator, David Frost, will speak to his EU counterpart, Michel Barnier, next week to try to agree a new timetable and method for the talks.

Barnier is recovering after contracting coronavirus but Johnson remains in intensive care with the disease.

Germany takes over the rolling EU presidency on 1 July from Croatia. It will be responsible for chairing key meetings and finding compromise positions on issues, including Brexit.

However, the German ambassador to the EU wrote that it would not be possible to look beyond the need to rebuild the EU economy.

He said: “From now on, the focus will be on the ability of the European institutions to act, crisis management, exit and reconstruction – possibly maintaining the EU integration itself. The success of our presidency will be measured against this.”

Clauss said other key issues would “inevitably be overlaid or completely receded” as the EU’s ability to act will “remain broadly limited until further notice”, according to the letter leaked to German magazine Der Spiegel.

Due to physical distancing rules, which are likely to remain even when lockdowns across Europe are lifted, the usual 21 boardrooms available for internal negotiations will be reduced to five. These will have to be shared with the EU’s foreign affairs wing, the European External Action Service.

“The council secretariat strongly recommends planning only with one video conference per day,” the letter said. It added that the EU was having to use a commercially available end-to-end encryption service.

Four new video conferencing systems set up in the European parliament are also said not to meet the required level of confidentiality.

The ambassador wrote: “Video conferences, even if they can be carried out, will not be able to replace physical meetings on an equal footing. No formal quorum, no marginalised conversations, no confidentiality of the negotiations, no interpreting. Difficulties in text work.”

A UK government spokesman said: “We remain fully committed to the negotiations and as we have been clear, discussions with the commission are continuing, with the aim of reaching an agreement by the end of the year. The transition period ends on 31 December 2020, as enshrined in EU law, which the Prime Minister has made clear he has no intention of changing.”