The first of three planes chartered by the government to bring Britons home from Libya has arrived from Gatwick after a technical fault had delayed it.
Britons at Tripoli airport are also being helped to board a plane chartered by oil companies for its employees that has spare capacity.
About 300 Britons remain in the Tripoli area, and 170 British oil workers are stranded in desert camps.
The government has announced a review of its evacuation response.
In a statement issued late on Wednesday, Foreign Secretary William Hague said: "We continue to do all we can to assist British nationals to leave Tripoli.
"We can confirm that we are currently assisting the majority of British nationals at the airport to board a chartered plane that has become available in Tripoli. This is being facilitated by our team on the ground.
"In addition, we expect a number of additional planes to arrive in Tripoli overnight to allow the departure of all the remaining British nationals at the airport.
"We are also continuing actively to pursue action to assist the return of British nationals elsewhere in Libya. Numerous [Foreign and Commonwealth Office] staff continue to work day and night to provide the best possible assistance to British nationals in Libya."
Mr Hague's statement came as the Foreign Office faced criticism over the speed of its response to the unfolding crisis, with some stranded Britons accusing the government of doing nothing to help them while other countries flew their citizens home.
Mr Hague added: "Given today's difficulties with reliably chartering private planes, I will establish a review, directly reporting to ministers, into the FCO's long-standing arrangements for aircraft evacuations.
"We need to know whether today was a coincidental series of unavoidable setbacks, or a systemic flaw."
The first Foreign Office flight, operated by Astraeus Airlines, took off from Gatwick at about 2200 GMT on Wednesday, after a technical hitch had caused a delay. It has capacity for 180 passengers.
A second flight is due to take off later and a third will leave on Thursday morning if needed.
One UK plane, chartered by oil companies including BP, arrived in Libya on Wednesday with some Foreign Office officials on board. The Foreign Office said that 105 of its 180 seats had been made available for British nationals who were not oil workers.
Earlier Mr Hague had said the government would send "as many planes as necessary" to bring Britons home. He had also said sending in military planes without permission would risk the safety of all those involved but it had not been ruled out.
Mr Hague has postponed a planned visit to Washington on Thursday, in which he would have met Secretary of State Hillary Clinton to discuss the situation in the Middle East, in order to remain in London and lead the Foreign Office's response to the crisis.
The Royal Navy frigate HMS Cumberland is also due to arrive off Libyan waters and help in evacuation efforts if needed.
Consular staff have also been deployed to Libya's border with Tunisia to help Britons who have made their way there.
Attacks and looting
Other countries, including France, Russia and the Netherlands have already evacuated some of their citizens.
More than 100 Britons have signed up for seats on the first Foreign Office-chartered flight. UK nationals wishing to register an interest should call the following numbers: 020 7008 0000 from the UK or 021 3403644/45.
The Foreign Office is advising against all but essential travel to Libya.
Although most of the 3,500 Britons who were living in Libya before the crisis are thought to have already left, those remaining have struggled to get home because scheduled flights have been cancelled.
British Airways and British Midland International have cancelled flights in and out of Tripoli for the past two days, and no services will operate on Thursday.
Most of the Britons in the isolated desert camps work for oil companies. They are struggling to make contact because the phone networks have been disrupted and their supplies from Libyan cities are running out.
"Some we know have been subjected to attacks and looting," said Mr Hague. "They are in a perilous and frightening situation."
Stranded oil worker Jim Coyle told BBC Radio 4's Today programme: "We are living every day in fear of our lives. We are living a nightmare here."
He said the workers had contacted British officials to warn them about the situation but nothing had happened.
"We have asked the British government to come here for days now and they are just totally ignoring us," he said.
"They don't reply to e-mails, they have cut off the phones to Tripoli. Basically we have been left without any protection whatsoever."
Chris Murphy and his wife arrived back in the UK on Wednesday afternoon. He said they had "fought their way" through crowds to get on the flight at Tripoli airport but when the plane took off, there were about 100 empty seats.
'Slow off mark'
A British woman said she and a Portuguese friend were helped out of the country by Portuguese embassy staff on Tuesday.
"There was nothing at all happening with the British Embassy. We felt very isolated and very out there on our own," the unidentified woman told BBC Radio 4's The World At One.
Shadow foreign secretary Douglas Alexander accused the UK government of being "slow off the mark" and said ministers should convene the emergency committee Cobra to co-ordinate the evacuation plans.
After a week of upheaval in Libya, protesters backed by defecting army units are thought to have almost the entire eastern half of Libya under their control.
The country's beleaguered leader Colonel Muammar Gaddafi, who has been in power for 42 years, has said enemies of Libya would be executed and vowed to fight to his "last drop of blood" rather than leave the country.