LONDON, Jan 13 (Reuters) - GlaxoSmithKline has handed back rights to an experimental drug for Duchenne muscular dystrophy to Dutch biotech firm Prosensa after it failed last year in a critical clinical trial.

Britain's biggest drugmaker said on Monday that Prosensa would now have full and unencumbered rights to drisapersen.

The move marks the termination of a 2009 collaboration agreement between the two firms.

Hopes for the drug slumped last September when it did not show a statistically significant improvement in the distance that patients could walk in six minutes compared with a placebo in a final-stage Phase III test run by GSK.