The 2009 surveillance of the new Turkish finance minister, Mehmet Simsek, and the possible targeting of up to 15 members of his delegation to London, laid out in a GCHQ document seen by the Guardian, is a startling illustration of how far the UK's appetite for electronic eavesdropping has strayed beyond traditional cloak and dagger.

Turkey is – and was at the time – a friendly power, a Nato ally with which the UK government has a good relationship. Some two years before this particular wiretap, Gordon Brown and the Turkish prime minister, Recep Tayyip Erdoğan, had signed a strategic partnership agreement in Downing Street, where Brown declared that bilateral ties were "strong and strengthening".

Furthermore, there is no suggestion that any of the officials named in the GCHQ document knew anything about security, far less had secret knowledge crucial to the defence of the realm.

On the contrary, the top-secret document about a G20 finance ministers' meeting in London starting on 2 September 2009 makes clear that the wiretappers' "reporting requirements" were to find out nothing more than Ankara's attitudes to financial regulation and reform, as well as Turkish "willingness (or not) to co-operate with the rest of the G20 nations".

The subject matter is the everyday talk of financial civil servants and central bankers. The apparent motive for the eavesdropping was to give British officials the slight negotiating edge of knowing what the Turks were thinking about financial reform before they showed their hand.

So why is GCHQ bugging them if the potential gains are so marginal? The answer seems to be because it can, both technically and legally.

In 1994, the Tory government managed to insert into the Intelligence Services Act a clause that allowed electronic surveillance "in the interests of the economic wellbeing of the United Kingdom in relation to the actions or intentions of persons outside the British islands".

The argument at the time was that national security also entailed economic security. The country would want to be prepared for sudden oil price shocks, for example.

But what was created was a capability without a constraint, and a new infinite list of foreign targets to eavesdrop on, no matter how marginal the advantage gained, as the 2009 Turkish GCHQ brief demonstrates.

Being a Nato ally seems to offer little protection against covert monitoring. The only boundary GCHQ appears to recognise is membership of Five Eyes, the tight coalition of western English-speaking states that share their signals intelligence: the US, UK, Canada, Australia and New Zealand. When it comes to eavesdropping on a national of a "second party", as Five Eyes members are called, that government has to be informed.

Anyone outside that select group is potentially fair game. In fact, the question the Turkish G20 document leaves unanswered is: if GCHQ was actively contemplating wiretaps on mid-level treasury officials in the hope of capturing their thoughts on the regulatory architecture of global finance, who would it not spy on?