Top institutional investors Fidelity International and Putnam Investments are warning the nine year rally in "junk" or high yield bonds is drawing to a close after the longest bull run on the market since 1998.

The development comes amid a boom in the volume of junk bond issues globally, in large part provoked by the soaring number of corporate takeovers and private equity-led leveraged buyouts and recapitalisations being financed in the market.