UPDATE : French investment bank BNP Paribas Capital Markets advised clients this morning

‘A Lab/Lib government is the least liked option by markets and would almost guarantee a downgrade of the UK sovereign. Moody’s suggested yesterday, when announcing that Greece and Portugal where due for a further downgrade (eventually giving Greece junk bond status), that the UK would not be downgraded while parties hold coalition talks. However, the implicit meaning of this statement suggests that the rating agency will look carefully into budget plans when deciding on the potential downgrade. We believe that a downgrade under a Lab/Lib government is more than likely since both parties agree that early expenditure cuts could harm the economy. The alternative could be that both parties agree on tax hikes to be implemented with the next budget. Both outcomes would be equally bearish for sterling. Hence, GBPUSD remains a sell.’