The EU plans to defy Tory Brexiters and retain its offices in London – the former Conservative central office at 32 Smith Square – as an outpost from which to communicate with British citizens after Brexit, leaked documents reveal.

High-profile Brexiters had called last year for the EU to hand back the large red-brick building that was previously Margaret Thatcher’s headquarters and the scene of her general election victories.

The European commission and the European parliament jointly purchased the Westminster building for £20m in 2010 after 50 years of Tory ownership, and renamed it Europe House.

Jacob Rees-Mogg MP, the chairman of the European Research Group, had suggested it would be a “wonderful” gesture of goodwill if, during the negotiations over the UK’s £39bn divorce bill, the building was returned.

But a leaked note on the administrative consequences of Brexit, seen by the Guardian, makes clear that the EU is keen to retain the advantages of the building and its plum position close to the houses of parliament in Westminster.

Klaus Welle, the European parliament’s secretary general, said the EU would need a position from which to champion the interests of its own citizens living in the UK, and to communicate its messages to the British.

“In the UK, parliament has established offices in London [the ‘Europe House’ shared with the European commission] and Edinburgh,” Welle, a former senior official in Angela Merkel’s German Christian Democratic Union party, wrote in the message to the chamber’s political leadership.

The EU’s office in Scotland was established close to Edinburgh Castle in 1975, the year of the UK’s first referendum on the then EEC, in order for Brussels to build contacts with local politicians. The office in London comprises eight floors and has an area of approximately 34,000 square feet.

“Although the UK is set to become a third country, as a former member state with many links to the EU, it nonetheless – and whatever the outcome of the negotiations – will remain a special and essential partner of the union. Most notably, as of now, more than 3 million non-British EU citizens reside in the United Kingdom,” Welle said.

“Both the London and Edinburgh offices offer unique advantages to particular parliament’s positions and serve as a vehicle for communicating with citizens.”

Welle continued: “The House of Europe in London is moreover ideally suited to host a wide range of external events, thanks to its location and meeting facilities, which may also serve as a means of engaging with civil society in the international setting that London will undoubtedly remain.

“The cost of the offices of parliament in the United Kingdom – buildings, staff and operations included – is within the normal range of EP [European parliament] liaison offices in larger member states. In light of the above, it is therefore proposed to continue parliament’s presence in the UK following the example of parliament’s Washington office.”

Welle’s note said, however, that the European parliament will have to examine its contracts with British suppliers when the UK withdraws from the EU.

While the ongoing Brexit negotiations might provide further clarity, he wrote: “The handling of contracts of parliament with British companies that will be in force when Brexit becomes effective is a further area of interest.

“One might have to analyse also contracts concluded with subsidiaries of British companies located in EU member states, such as the internet and voice data supply contractor of parliament ... A process of continuous legal and operational analysis has been initiated by the secretary general in the legal service, on the basis of a screening exercise.”

In January 2016, BT announced two new contracts with the European commission worth £24m to deliver public and private cloud services across 52 major European institutions, agencies and bodies – including the European parliament, the European council and the European Defence Agency. “We firmly believe that BT should continue to retain its existing EU contracts and be able to bid for new EU contracts post-Brexit,” the company said on Thursday.

The Liberal Democrat MEP Catherine Bearder said: “Brexit is bad for British companies doing business anywhere in the EU. At the moment British companies have the right to tender for EU contracts – that will be lost when we leave.”

A European parliament spokesman said the document was drafted in the runup to a “first exchange of views on the administrative consequences of Brexit”.

He added that MEPs had “decided to establish a working group to consider all possible consequences thoroughly and recommend measures including regarding existing European parliament’s representation in the UK after Brexit. No final decision were taken at this stage regarding the Liaison offices of London and Edinburgh.”