Britain's Charles III will travel to Germany on Wednesday for his first state visit as king, with the trip billed as "an important European gesture" to maintain strong ties after Brexit.

The British Union Jack was flapping alongside the German and European Union flags along Berlin's central Unter den Linden avenue, which leads to the Brandenburg Gate, where Charles will receive a formal welcome.

German President Frank-Walter Steinmeier and German first lady Elke Buedenbender will greet Charles and Queen Consort Camilla with military honours at the landmark, the first time it has provided a backdrop for receiving a state guest.

Dozens of police were milling around the gate in the morning, ahead of the king's arrival later in the day.

During his three-day visit, Charles will become the first monarch to address the German parliament, with a speech on Thursday, before travelling to the port city of Hamburg on Friday.

The decision to call on close neighbours first is widely seen as an attempt to build post-Brexit bridges, with Steinmeier calling it "an important European gesture".

The choice of Berlin for Charles's first visit, after a planned trip to Paris was postponed, showed Germany was a "key partner" for Britain as it sought to reset its relations with the EU, daily Frankfurter Allgemeine Zeitung wrote.

The German public will get a chance to come face-to-face with the royal couple at the Brandenburg Gate on Wednesday, although with space for only around 1,500 spectators, well-wishers have been advised to turn up early.

- Public interest -

"The British royal family garners a lot of interest" in Germany, said Michael Hartmann, a sociology professor at Darmstadt Technical University, noting the bestseller status of the tell-all memoir by Charles's son Prince Harry.

The fascination with the House of Windsor has not dimmed since the death last year of Elizabeth II, Hartmann told AFP.

The late queen first visited Berlin in 1965 when the city was divided between a capitalist West and communist East, a trip that was seen as a key step in postwar reconciliation.

Charles himself has been a regular visitor to Germany, with the British Embassy in Berlin saying that he has been to the country 40 times.

"We will always be friends, partners and associates," the then-Prince of Wales told the Bundestag in 2020 switching between English and German.

Charles's command of German may be a nod to the British royal family's roots in Germany notably through Charles's great-great-great-grandfather Albert of Saxe-Coburg and Gotha, queen Victoria's husband.

- Organic farm -

Following Wednesday's welcome ceremony, Charles and his wife will travel to Steinmeier's Bellevue Palace, where he will host a state banquet in the evening.

On Thursday, Charles will speak to Chancellor Olaf Scholz and meet refugees recently arrived from Ukraine.

Steinmeier and the royal couple will then visit a British-German military unit as well as an organic farm in the surrounding state of Brandenburg.

In Hamburg on Friday, the king, who has for decades pushed an environmental agenda, is due to tour a renewable energy project.

He will also have another opportunity to meet with the German public in the port city.

And in what Bild daily has described as a gesture of deep significance, Charles will in Hamburg commemorate victims of 1943 air raids targetting the city.

The bombings, code named Operation Gomorrah, were carried out by night by the British air force, and by day by US forces.

During their time in Berlin, the royal couple will stay next to the Brandenburg Gate at the famous Adlon Hotel, which has regularly played hosts to celebrities passing through the German capital.

Police are out in force, with about around 1,000 officers deployed on Wednesday and Thursday to ensure security.

Charles was initially supposed to travel to France before heading immediately to Germany, but his trip was postponed in the wake of violent pension reform protests.

The visit was intended to highlight warming Franco-British relations but will now have to wait for another date.