"Day 18,” the 16-year-old said in a message on Twitter. “We’re speeding towards Europe! Estimated time of arrival right now is Tuesday morning. We’ll be arriving at Doca de Alcantara, Lisbon. We are all looking forward to see you there!”

In a statement sent to Lusa on Thunberg’s message, ZERO (Sustainable Land System Association) stated that the sailboat La Vagabonde will dock at The Dock of Alcântara at the end of the pontoon on the River Tagus, next to the Gare Marítima de Alcântara.

Upon her arrival, the young activist is to be received by the mayor of Lisbon, Fernando Medina, and a news conference is also scheduled on the site, the association said.

COP25, which is scheduled for 2 to 13 December, was on 1 November transferred urgently to Madrid from Chile, the original planned host, after the latter announced that it would no longer be organising it because of the social unrest roiling the country.

The transfer was proposed by Spain’s Socialist prime minister, Pedro Sánchez, to "ensure" that the meeting went ahead, despite the enormous logistical challenges this will entail: setting up in just one month an event that is to see the participation of 25,000 people from some 200 countries, including diplomats, representatives of non-governmental organisations, and scientists.

The climate change summit takes place annually in a different region of the world. Last year it took place in Poland.

Initially, the conference was to have taken place in Brazil but in November 2018 the newly elected president of that country, Jair Bolsonaro, gave up the right to organise the event, citing "fiscal and budgetary restrictions", and Chile took over.

Thunberg had crossed the Atlantic by boat to attend an earlier United Nations summit in New York, convened by the UN secretary-general, António Guterres, in September and then the COP25 in Chile. The unexpected change of venue forced her to return to Europe, this time in a catamaran, to arrive in Madrid in time without having to catch a plane, and with a stop planned in Lisbon.