The British counterpart to the NSA is the GCHQ. I gather from the article that both agencies were part of this operation.  From the Guardian:

Foreign politicians and officials who took part in two G20 summit meetings in London in 2009 had their computers monitored and their phone calls intercepted on the instructions of their British government hosts, according to documents seen by the Guardian. Some delegates were tricked into using internet cafes which had been set up by British intelligence agencies to read their email traffic.

Government apologists, in brushing off recent revelations of NSA spying repeatedly point out that all nations do it, so the fact that the U.S. is doing it should come as no surprise.  They then go into the awfulness of Snowden’s leaks and how damaging it is to national security.  It would appear that Britain’s scruples aren’t anymore high minded than those of the U.S.   Neither Britain nor the U.S. are at war with any of the G20.

One document refers to a tactic which was “used a lot in recent UK conference, eg G20”. The tactic, which is identified by an internal codeword which the Guardian is not revealing, is defined in an internal glossary as “active collection against an email account that acquires mail messages without removing them from the remote server”. A PowerPoint slide explains that this means “reading people’s email before/as they do”.

Furthermore, by logging keystrokes at the fake internet cafes, Britain was able to secure login information permitting future intelligence gathering on those accounts.

The September meeting of finance ministers was also the subject of a new technique to provide a live report on any telephone call made by delegates and to display all of the activity on a graphic which was projected on to the 15-sq-metre video wall of GCHQ’s operations centre as well as on to the screens of 45 specialist analysts who were monitoring the delegates.

“For the first time, analysts had a live picture of who was talking to who that updated constantly and automatically,” according to an internal review.

And analysts were able to forward that intelligence to British representatives in near real time, giving them a decided advantage in negotiations. The newly released data apparently indicates that then PM Gordon Brown sanctioned the spying.

This latest revelation ought to make quite the splash as the UK is preparing to host, on Monday, the G8, all of whom attended the G20 meeting in 2009.

It is likely to lead to some tension among visiting delegates who will want the prime minister to explain whether they were targets in 2009 and whether the exercise is to be repeated this week.