The two-day summit at a luxury resort in Northern Ireland, which starts on Monday, will also centre on the Syria conflict.
During pre-G8 talks at his Downing Street office, Cameron reached an agreement with territories such as the Cayman Islands and the crown dependencies of Jersey, Guernsey and the Isle of Man, which are often seen as tax havens.
They agreed a series of actions aimed at promoting transparency and exchange of information between tax jurisdictions.
“It is a very positive step forward and it means that Britain’s voice in the G8 and the campaigning on this issue around the world for proper taxes, proper companies and proper laws … will be stronger,” Cameron said afterwards.
“Let’s be clear why this tax issue matters. If companies don’t pay their taxes or individuals don’t pay their taxes we all suffer as a result.”
He said his programme for the Group of Eight world powers was about “proper companies, proper taxes and proper global rules ensuring that openness delivers the benefits it should for rich and poor countries alike.
“Aid is important but these things matter just as much. Now is the time. This is the agenda. The world should get behind it.”
There was progress late on Friday towards what Cameron has admitted would be the biggest prize of the summit – the start of formal negotiations between the European Union and the United States on a free trade agreement.
EU trade ministers finally thrashed out an agreement on how to negotiate for a deal, after meeting a French demand to exclude the key audiovisual sector.
But the Syrian conflict looks set to dominate the talks after Washington upped the ante by pledging military aid to rebels seeking to oust President Bashar al-Assad.