Merger and acquisition activity has had a disastrous effect on US blue-chip corporates so far this quarter, prompting three ratings downgrades for every upgrade - but it has had a positive net effect on high-yield companies.

Since the beginning of July mergers and acquisitions have led to 38 investment-grade companies receiving a ratings downgrade - affecting a total of $88bn (€100bn) of debt, according to Moody's, the rating agency. Just 12 companies received upgrades and this impacted only $11bn of debt.