Final answer:
European countries like Great Britain had to agree to adopt a common tariff policy, renounce agricultural privileges, and nullify existing trade arrangements to be part of the Common Market. This agreement indicated substantial policy changes and reshaping of international trade relations.
Step-by-step explanation:
In order to be a part of the Common Market, also known as the European Economic Community (EEC), European countries such as Great Britain had to agree on several substantial changes. One key aspect was the commitment to a common tariff policy, meaning they would have to let go of any preferential trade deals, notably those with the Commonwealth. Secondly, they would need to accept the common agricultural policy, which would mean no more privileges for their own agriculture sector. Moreover, the member countries had to treat their existing trade arrangements with countries in the free trade area as null and void. This requirement was particularly challenging as it involved significant alterations to well-established trade relationships and domestic policies.
Admitting Great Britain into the EEC would substantially transform the nature and volume of the European community, as emphasized by French President Charles de Gaulle’s stance on the matter. His eventual veto stemmed from concerns about the UK's ability and willingness to fully embrace the strict conditions that membership to the Common Market entailed.