G20 rules on tax avoidance would limit Britain's ability to offer tax breaks to companies, even after it left the EU, the German minister argued.
At their four-day conference, the party will also debate plans to toughen up the rules on tax avoidance and change media ownership laws.
"First, clamp down hard on avoidance – with specific measures, including on stamp duty, and a tough, new, general anti-avoidance rule.
Count on avoidance products and services to increase, not decrease.
The depressed vigilance may prompt an inhibitory influence on avoidance.
The EU investigation is believed to be the first time the commission has resorted to the rules on state aid to try to curb tax avoidance.
The prime minister, who will also hold talks about Syria in Downing Street on Sunday with the Russian president, Vladimir Putin, before travelling to Northern Ireland, believes that significant steps can be taken on tax avoidance at the G8 after the EU agreed new rules on exchange of information.
Hard-hitting new measures aimed at those who sell tax avoidance arrangements are also in the pipeline, as well as stricter rules on informing HM Revenue and Customs HMRCC) about new schemes.
Countries are urged, for instance, to tighten rules on "transfer pricing" of transactions between subsidiaries in different countries; or to strengthen their "general anti-avoidance rules".
The European Commission will propose rules on Tuesday to force major companies to publish details of where they make profits and where they pay tax, as it moves to clamp down on tax avoidance following the Panama Papers revelations.
