Three people were arrested on Thursday following a scuffle with the police outside the Ecuadorean embassy where WikiLeaks founder Julian Assange has taken refuge to avoid extradition to Sweden to face allegations of sexual offences.
The scuffle broke out as Britain and Ecuador were locked in a diplomatic impasse over Assange.
Ecuador is due to announce its decision in capital Quito on Assange's application for asylum, but Britain said it would seek to extradite him even if he were granted asylum.
Britain said it will not offer "safe passage" to Assange if he is granted asylum by Ecuador. Assange is liable to be arrested the moment he steps out of the embassy.
Given Ecuador's hardening of position after Britain delivered what Quito considered a "threat" to enter its embassy in London, it is widely expected that Assange's asylum application will be successful, which will set in motion another chain of events related to his extradition.
Ecuador foreign minister Ricardo Patino had on Wednesday said: "We want to be very clear, we are not a British colony. The colonial times are over".
Britain says it is under legal obligation to extradite Assange to Sweden while Ecuador insists that British authorities entering the embassy would violate the Vienna convention and would be considered a hostile act.
Britain believes that it can withdraw diplomatic immunity and then enter the embassy under the Diplomatic and Consular Premises Act, 1987.
Reports from Quito quoted extracts of the British embassy letter: "You need to be aware that there is a legal base in the UK, the Diplomatic and Consular Premises Act 1987, that would allow us to take actions in order to arrest Assange in the current premises of the Embassy".
The letter added: "We need to reiterate that we consider the continued use of the diplomatic premises in this way incompatible with the Vienna Convention and unsustainable and we have made clear the serious implications that this has for our diplomatic relations".
An Ecuadorean government spokesman said: "We are deeply shocked by the British government's threats against the sovereignty of the Ecuadorean Embassy and their suggestion that they may forcibly enter the embassy".
He added: "This a clear breach of international law and the protocols set out in the Vienna Convention. This stands in stark contrast to the escalation of the British government today with their threats to break down the door of the Ecuadorean Embassy".
In London, a Foreign Office spokeswoman said: "The UK has a legal obligation to extradite Assange to Sweden to face questioning over allegations of sexual offences and we remain determined to fulfil this obligation".
She added: "We have an obligation to extradite Assange and it is only right that we give Ecuador the full picture. Throughout this process we have drawn the Ecuadoreans' attention to relevant provisions of our law, whether, for example, the extensive human rights safeguards in our extradition procedures, or the legal status of diplomatic premises in the UK. We are still committed to reaching a mutually acceptable solution".