It was reported on Tuesday (August 14) that Assange is expected to be granted political asylum in Ecuador, with a formal decision expected later on Thursday (August 16).
The Wikileaks founder is fighting extradition to Sweden, where he is wanted for questioning in regards to sexual assault charges. Assange denies the allegations.
Assange's mother has expressed her concern that if her son were to be extradited to Sweden he could then face extradition to the US, where he could potentially face the death penalty under the charge of espionage for his role in the leaking of US military and diplomatic cables.
Ecuador's foreign minister Ricardo Patino said at a press conference: "Today we received from the United Kingdom an express threat, in writing, that they might storm our Embassy in London if we don't hand over Julian Assange. Ecuador rejects in the most emphatic terms the explicit threat of the British official communication.
"If the measure announced in the British official communication is enacted, it will be interpreted by Ecuador as an unacceptable, unfriendly and hostile act and as an attempt against our sovereignty. It would force us to respond. We are not a British colony".
Police officers have been seen guarding the door to the Ecuadorian embassy in Knightsbridge.
The UK would have to revoke the embassy's diplomatic status in order to enter, using the Diplomatic and Consular Premises Act 1987 to declare that the embassy is no longer treated as Ecuadorian soil.
Additionally, if Assange is granted asylum by Ecuador, he will still have to cross UK soil to leave the country.
The Foreign Office said on Wednesday (August 15) that it is "still committed to reaching a mutually acceptable solution" and noted that it has a "legal obligation" to extradite Assange.
"Throughout this process we have drawn the Ecuadorians' attention to relevant provisions of our law, whether, for example, the extensive human rights safeguards in our extradition procedures, or to the legal status of diplomatic premises in the UK," a departmental spokesperson explained.
Assange sought refuge at the Ecuadorian embassy in June.