The prime minister and his British counterpart Boris Johnson announced the deal on Tuesday after sealing an in-principle agreement over dinner overnight.

"I said we would wait for the right deal, and I think we've got the right deal between the UK and Australia," Mr Morrison told reporters outside 10 Downing Street.

"Our economies are stronger by these agreements. This is the most comprehensive and ambitious agreement that Australia has concluded."

The deal will pave the way for more Australians to live and work in Britain and offer exporters more market options.

It will also scrap a requirement for British backpackers to work on Australian farms before extending their visas.

This could create issues for producers who need seasonal workers to pick crops and regional pubs and clubs who rely heavily on working holiday makers.

But the Nationals say they have secured an agreement to guard against labour shortages, which will be separate to the UK trade deal.

Mr Morrison said the number of Australians expected to live and work in Britain would be demand-driven.

"We go into this boldly and confidently. It's a great opportunity for people of both countries," he said.

Several key sticking points needed to be overcome before the initial agreement could be reached.

Agriculture proved to be the major obstacle with squabbles over Australian lamb, beef and dairy products.

Trade Minister Dan Tehan talked up the UK pact during a coalition party room meeting in Canberra.

"Our aim is to have the best deal outside of the deal with New Zealand. The negotiations have been hard fought," he said

Agriculture Minister David Littleproud was slightly more circumspect, telling reporters it was another feather in Australia's cap, but not one that would rival trade with Asia and the Middle East.

The UK is Australia's fifth largest trading partner, with two-way goods and services valued at $36.6 billion, and its second biggest investment partner.

Mr Tehan said Australian dairy producers were limited to an import quota of 44 grams of cheese per person every year, while the average Briton consumed 125 grams of cheese per week.

"What's more, they are missing out on eating the best lamb chops, the best steak in the world and washing it down with the best glass of Australian wine that you could imagine," he said.

Ahead of the formal announcement, the prime minister made a free trade pitch to business leaders from both countries.

He said it was the most substantial deal done since the UK withdrew from the European Union.

"As the United Kingdom moves into a completely new generation of their trading relationships with the world, who better to start that journey with than Australia?" Mr Morrison said.

He described the UK joining the common European market decades ago as a devastating blow to Australian producers.

"The Brexit that has occurred is an opportunity for us to pick up where we left off all those many years ago and to once again realise the scale of the trading relationship we once had."

Deputy Opposition Leader Richard Marles said Labor had concerns about agricultural exports and visa conditions for farm workers, which the party would work through in time.

Australian Associated Press