The declaration issued at the Ise-Shima summit in Japan is the latest in a series of warnings by international bodies of the dangers of a Leave vote in the June 23 referendum on the UK's EU membership.
The warning comes as it emerged on Friday that EU officials and diplomats have launched a round of confidential discussions to prepare a coordinated response to a possible British vote to leave.
The G7 document, released on the final day of the two-day summit, describes the referendum as one of a number of "potential shocks of a non-economic origin" which could hit world growth.
"UK exit from the EU would reverse the trend toward greater global trade, investment and the jobs they create and is a further serious risk to growth," warned the document, agreed by leaders of the UK, US, Japan, Germany, France, Italy and Canada.
As the summit wound up, Reuters and Frankfurter Allgemeine Zeitung reported that, with uncertainty continuing over the outcome of the UK referendum, EU officials and diplomats have now launched a round of confidential discussions to prepare a coordinated response to a possible British vote to leave.
A source said it was intended to be followed by others on specific topics.
"The idea is to have everything ready for the Monday," an EU source was quoted by the two news organisations as saying.
"There will be a lot of talk about the show going on," said another source.
"There will be expressions of regret, of respect for the wishes of the British people, and probably some dire warnings about consequences to discourage others from doing the same."
It was also reported that Ivan Rogers, the UK's ambassador to the EU, has been "coaching" EU politicians on what they should and should not say during the final run-up to the referendum, a period when UK civil servants must remain silent on the issue.
Rogers is said to have "shifted the tone" this week by encouraging leaders to make the case that there will be serious consequences if the UK votes 'No' - a definitive split from the EU, not another renegotiation of membership terms.
On Wednesday, Rogers met German MEP Manfred Weber, the leader of the EPP group, Parliament's largest political group
According to EPP group spokesperson Pedro López de Pablo, Weber told Rogers that "we are ready to do whatever you want - a speech, an article - we are at their disposal to help the 'Yes' campaign."
Parliament President Martin Schulz is also said to have been in regular contact with Rogers on how best to weigh into the Brexit discussion.
Schulz's spokesperson Armin Machmer was quoted as saying, "As friends of the UK we have a responsibility to voice our opinion if asked and we have done so and will do so. We share his view that there will be consequences and no turning back."
Elsewhere, in a speech to the European Parliament this week, Dutch King Willem-Alexander told MEPs that he would prefer to see the UK stay in the EU, arguing that the "European bouquet" is not complete "without the English rose."
Vote Leave, meanwhile, has claimed that EU procurement rules, aimed at ensuring that firms are not discriminated against in competition for public contracts across the EU, have cost more than €9bn over the last five years.
A BMG Research online poll found that 45 per cent supported Britain leaving, unchanged from a similar poll last month, while backing for a remain vote rose one point to 44 per cent.
However, a Survation telephone poll showed 44 per cent would vote to stay in the EU compared with 38 per cent who would seek to leave - little changed from last month's poll.