Julian Assange, the WikiLeaks founder, faces immediate arrest the moment he leaves the Ecuadorean embassy in central London, even if he is granted political asylum, police and British government officials have indicated.

He spent his first full day confined to the small Ecuadorean embassy in Knightsbridge on Monday, unable to move as his lawyers, diplomats and government officials grappled with the implications of his request for political asylum to avoid extradition to Sweden.

His surprise move on Tuesday evening has left some of Assange's most prominent supporters facing the loss of up to £240,000 in bail money provided to secure his freedom when he first faced extradition proceedings. Supporters including the activist Jemima Khan, film director Ken Loach and publisher Felix Dennis posted cash security of £200,000 with Westminster magistrates court with a further £40,000 as promised sureties when Assange was freed in December 2010.

Assange's gambit came after he lost the last of his many appeals against removal to Sweden to be questioned over sex crime accusations, for which he has been on police bail since December 2010.

He had been driven by what he called "really terrible choices" to make the last-ditch move, his New-York-based lawyer told the Guardian, believing it highly likely that the US intends to seek his onward extradition from Sweden on espionage charges over the WikiLeaks cable releases. "What he was facing was never seeing the light of day for the next 40 years," said Michael Ratner, of the Centre for Constitutional Rights, which represents the WikiLeaks founder in the US.

The Swedish lawyer representing the Australian's two alleged victims said the latest attempt to avoid extradition was "a tragedy for the women". Claes Borgström said the women were "frustrated" and "disappointed".

"The tragedy is that he doesn't take his responsibility. He should have come to Sweden," Borgström added. Assange has not been charged in Sweden and denies the allegations.

The Foreign Office has confirmed that the first-floor embassy – consisting of the ambassador's office and some other small rooms, one of which Assange is said to be occupying – is diplomatic territory and that while there Assange is "beyond the reach of police". But officials are adamant that even if Ecuador grants him asylum or a diplomatic passport, he faces immediate arrest the moment he steps out of the front door for breaching his bail conditions.

Even taking Ecuadorean nationality would not help, officials stressed, as he could not claim asylum in what would be then his own embassy. He is understood to have surrendered his own Australian passport to British authorities.

Metropolitan police officers attended the embassy , after confirming that an arrest warrant had been issued for breach of the Bail Act.

Anna Alban, the Ecuadorean ambassador, said she had met Foreign Office officials, and she explained "that the decision on Mr Assange's application would be assessed by the department of foreign affairs in Quito and would take into account Ecuador's long and well established tradition in supporting human rights."

Her government would be seeking "a just and fair solution to this situation", she said.

Ricardo Patino, Ecuador's foreign minister, said on Tuesday that the WikiLeaks founder had written to the country's president, Rafael Correa, to ask for asylum. Assange interviewed him last month for his TV show The World Tomorrow, broadcast on the Russian state-sponsored channel Russia Today, in which the president told him: "Welcome to the club of the persecuted."

Ecuador is the only country to have expelled its US ambassador over the WikiLeaks cable revelations. It was during the filming of this interview that an offer of asylum was made, the Associated Press reported, quoting a woman who had been present during the interview but had spoken on condition of anonymity. It did not say whether the offer was personally made by Correa.

Ratner said Assange's move had been prompted purely by his fears of future prosecution in the US rwhere a secret grand jury has been empanelled into the WikiLeaks founder, ather than a desire to avoid the Swedish accusations. "Had the US come forward and said, 'We will not prosecute Julian Assange, I think he would be in Sweden tomorrow to deal with the allegations of the sex crimes. It was not about that at all. It was only about the US."

Marianne Ny, the Swedish prosecutor who had secured Assange's extradition in the supreme court in London, is following developments closely, said her spokeswoman, Britta von Schoultz. Ny will not comment on Assange's decision to seek asylum at the Ecuadorean embassy until he is returned to Sweden. "It is solely a case for the British authorities to handle," Von Schoultz said. "When it becomes a case for the Swedish prosecutor, she will comment on the case."

A leading criminal lawyer said those supporters who had stood bail for Assange would have to persuade the courts why they should not forfeit their money. Oliver Lewis, partner at Powell Spencer and Partners Solicitors, said: "There would have to be a pretty good reason … Usually the court says thank you very much, you have lost your money."

Vaughan Smith, the founder of the Frontline Club, was asked to offer surety of £20,000, and was unclear whether he would forfeit it. "The money is important because it relates to the welfare of my wife and children, but they don't feel they are at risk of being sent to America," he said. "I remain a supporter and it is important we recognise [Assange] is a western dissident. There are a lot of people who believe the work he did at Wikileaks was in the public interest.""

Khan confirmed on Twitter that she had also posted bail money for Assange. "I had expected him to face the allegations," she said. "I am as surprised as anyone by this."In his asylum request, Assange accused the Australian government of making an "effective declaration of abandonment", refusing to make interventions on his behalf to the Swedish or US authorities.

At a press conference, Julia Gillard, the Australian prime minister, said: "Mr Assange's decisions and choices are a matter for Mr Assange We, our officials, our consular officials, will be in contact with him and also with Ecuador in London about this, but his decisions in relation to this matter are for him to make.."But the Australian Green party attacked its government's response as "feeble", saying it amounted to "malign indifference".During Correa's six years in office, his administration has continued to welcome foreign refugees, of which more than 95% have fled the protracted civil war in Colombia. On Monday, António Guterres, the UN high commissioner for refugees, commended Ecuador for being an "example of solidarity" by receiving Latin America's largest refugee population.