Australia is ready to secure a free trade deal with Britain as soon as it formally leaves the European Union, Prime Minister Malcolm Turnbull says after talks with British Prime Minister Theresa May in London.

Key points: Mr Turnbull says UK, Australia committed to depend trade, investment relationship post-Brexit

Mr Turnbull says UK, Australia committed to depend trade, investment relationship post-Brexit UK Prime Minister Theresa May says the relationship is worth $18 billion

UK Prime Minister Theresa May says the relationship is worth $18 billion Mr Turnbull has visited scene of London Bridge attack, will visit Queen Elizabeth II

Speaking to reporters after the leaders met to discuss security and trade, Mr Turnbull was asked how long it would take to secure a new trade deal after Brexit.

"As soon as possible, if we move quickly," he said.

"Australians are fleet of foot, we don't muck around … we will move as quickly as the UK is able to move."

Ms May said securing a free trade deal between the UK and Australia remained a priority of her Government.

"We've both made clear our intention to continue to deepen our trade and investment relationship as the UK leaves the EU," she said.

"Our Brexit negotiations have started well and I've made clear to Prime Minister Turnbull that an ambitious and comprehensive bilateral trade deal with Australia remains a priority for the UK.

"Today we've talked about how we can step up our cooperation in a range of areas including security and defence, trade and investment and on the world stage."

Ms May said the UK and Australia were "investors in each other's success" and their growing trade relationship was worth close to $18 billion.

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Mr Turnbull said there were no two countries that trusted each other more than Australia and the UK.

"So when your institutions are attacked, we feel that ours are too. When Britain is attacked by terrorism we feel that we are attacked as well," he said.

During Mr Turnbull's visit, the two leaders also visited London's Borough Market, the site of an attack last month that killed eight people, two of them Australians.

Three attackers in a van ran down pedestrians on London Bridge, then stabbed people in the nearby market before being shot dead by police.

Mr Turnbull said he was moved to visit the scene of the attack, which included meeting two of the police officers who responded.

"The three of us did our best not to burst into tears," he said.

Ms May and Mr Turnbull also spoke to business owners in the food market, which is surrounded by restaurants, bars and cafes.

Mr Turnbull will also meet with Queen Elizabeth II during his visit.

The two leaders visited London's Borough Market, the site of an attack that killed two Australians last month. ( AP: Niklas Hallen/pool )

ABC/wires