Julian Assange has been living inside the building since June 2012 to avoid extradition to Sweden, where he faced questioning about sex crime allegations, which he always denied.
Although the Swedish investigation was dropped last May, Mr Assange still faces arrest in the UK should he step outside the embassy for breaching his bail conditions.
Today, Mr Duncan slammed the 46-year-old Australian during a question-and-answer session on foreign affairs in the House of Commons.
He told MPs: “It’s of great regret that Julian Assange remains in the Ecuador embassy.
“It’s about time that this miserable little worm walked out of the embassy and gave himself up to British justice.”
Mr Assange fears extradition to America, insisting the real reason for his pursuit by the authorities is the publication of US diplomatic and military secrets.
And, in an emailed response to Mr Duncan’s comments, he insisted Britain was to blame for the “disgraceful impasse”.
He said: “Britain should come clean on whether it intends to extradite me to the United States for publishing the truth and cease its ongoing violation of the UN rulings in this matter.
”I have already fully served any theoretical (I haven’t been charged) ‘bail violation’ whilst in prison and under house arrest.
“So why is there a warrant for my arrest?”
Mr Assange considers himself to be under arbitrary detention in the Ecuadorean embassy.
That is rejected by British authorities who say he voluntarily went into the building and could leave anytime if he were prepared to face up to the consequences of his actions.
Last month, a British judge refused to halt legal proceedings against Mr Assange for jumping bail and said he was “a man who wants to impose his terms on the course of justice”.
British police ended their permanent guard on the embassy in October 2015 but said they would maintain “covert tactics” to arrest him if he left.
At the time, they said £12.6million had been spent on guarding the embassy.
Ecuador’s president is also becoming fed up with the long-running saga.
President Lenin Moreno said in January that granting asylum to the controversial figure had become “more than a nuisance”.
After granting him naturalisation, the Ecuadorian government asked Britain to recognise Mr Assange as a diplomatic agent, which would have given him immunity to leave the embassy without being arrested. However, the UK denied the request.
Mr Moreno said: “This would have been a good result, unfortunately, things never turn out as the Foreign Ministry has planned and at this moment the problem is still there.”