Britain's last AAA credit rating is now hanging by a thread

Standard & Poor's, the last of the major credit ratings agencies to give the UK an AAA-grade assessment, just warned that the outlook isn't good.

The UK's credit rating hasn't been cut yet, but it's now on outlook negative - a warning that the next action is much more likely to be a cut than anything else.

Fitch and Moody's, the two other big ratings agencies, already cut the UK to AA+ and AA1 respectively.

Here's S&P:

The outlook revision reflects our view that the decision of the newly elected Conservative majority government to hold a referendum on the U.K.'s EU membership by 2017 represents a risk to growth prospects for the U.K.'s financial services and export sectors, as well as the wider economy. We believe a possible U.K. departure from the EU also raises questions about the financing of the U.K.'s large twin deficits and its high private short-term external debt.

Chancellor George Osborne used to argue that the UK's AAA rating was a symbol of its success - you don't hear him mention that much any more.