FRANKFURT (Reuters) - Euro zone banks are set to tighten access to corporate credit for the first time in two and a half years as they become more wary of risk and negative interest rates eat into their profit, a European Central Bank survey showed on Tuesday.

The ECB's ultra-easy policy - including bond purchases, free loans to banks and a charge on deposits - has fueled a recovery in euro zone lending but banks' margins have started to suffer, raising concerns about the negative side effects of sub-zero rates.

Banks stopped easing credit standards for enterprises in the three months to September and a narrow majority expects to tighten them in the current quarter, an ECB survey of 141 lenders showed.

The banks, polled last month, cited lower risk tolerance as a tightening factor when deciding which companies would get a loan or credit line in the three months to September.

They also voiced their discomfort with the ECB's negative deposit rate, effectively a charge on banks' excess deposit with the central bank.

A large majority of banks said the charge had a negative impact on their lending rates and margins over the past six months. Some even said it was leading to an increase in their charges to corporate customers over and above interests.

Companies' demand for credit grew less than banks had expected in the third quarter of the year, with banks in Spain and Italy actually reporting a fall.

The ECB's ultra-low interest rates were seen as the main contributing factor behind the increase in demand in the third quarter, with mergers and acquisitions activity and debt refinancing also contributing.

On balance, lenders still expect demand to grow in the last three months of the year.

(Reporting by Francesco Canepa; Editing by Balazs Koranyi and Alison Williams)