Julia Gillard has warned "good" free trade agreements take years to broker and are "very difficult" to seal, as the UK continues "preliminary" talks with Australia on a new deal.

The former Australian prime minister, speaking at London's Institute of Government think tank today (13 October), compared such negotiations to selling a car.

"[If you] sell your car for a dollar, you can always get a deal," Gillard said. "When you actually look at the length of time for trade agreements, it's measured in years.

"The Canadian agreement [with the EU] has taken seven years and it is partial on services.

"So if the world is going to move to an era of rapid trade agreements, maybe it would be a good era to live in, but I don't think contemporary signs are very good."

The comments come after International Trade Secretary Liam Fox and his Australian counterpart Steven Ciobo launched a working group in September to discuss a potential agreement between the countries after the UK splits from the EU.

Ciobo is also in talks with officials in Brussels in a bid to strike a free trade agreement with the EU.

Meanwhile, the UK government is facing renewed opposition over its plans to leave the economic and political bloc amid fears of "hard Brexit".

The concerns came after Theresa May's administration promised to prioritise immigration curbs in their negotiations with Brussels.

But the prime minister has ruled out adopting an Australian-style visa system, an initiative backed by the Vote Leave campaign and Foreign Secretary Boris Johnson. Gillard also urged caution over the issue, stressing "context is everything".

"We run a very sizable immigration programme. Our current permanent migration programme is running at 190,000 people a year," she said.

"You're three times bigger than us population wise so – rough maths – [that would] be 600,000 a year [in the UK]."

Gillard's visit to the UK capital comes just a week after fellow former Australian Prime Minister Tony Abbott spoke at the Conservative Party conference in Birmingham.

The right-winger spoke to IBTimes UK ahead of the event, revealing that he thought Brexit means "freedom".

"It means Britain controlling its own destiny, obviously it needs to interact with the other countries of the world, but it will be able to do it as independent actor, rather than needing to get permission in so much of what it does from Brussels," he said.

Abbott had urged UK voters to back Remain before the EU referendum on 23 June.