British Telecom, the UK telecoms provider that launched a benchmark long-dated sterling corporate bond three years ago, has seen its debt undergo a roller coaster ride that has left it trading more expensively than when it was launched - despite numerous rating downgrades.

BT's £600m (€980m) bond due December 2028, launched on April 28 1999, was the largest ever Eurosterling corporate bond at the time. It set a benchmark for other UK companies at the long end of the yield curve and demand was strong enough to prompt a £100m increase from the initial £500m that was expected.