By Ben Hirschler

LONDON (Reuters) - Britain is to strengthen the law on disclosing drug trial results following a four-year inquiry into GlaxoSmithKline Plc's (GSK.L: Quote, Profile, Research) delay in reporting data linking its antidepressant Seroxat to suicide risk in teenagers.

"We will take immediate steps to ensure the law is strengthened, so that there can be no doubt as to companies' obligations to report safety issues," Kent Woods, Chief Executive of the Medicines and Healthcare products Regulatory Agency (MHRA), said in a statement on Thursday.

The watchdog criticized Europe's biggest drugmaker for not handing over clinical data earlier but said criminal charges would not be brought because legislation in place at the time was not sufficiently strong.

"I remain concerned that GSK could and should have reported this information earlier than they did. All companies have a responsibility to patients and should report any adverse data signals to us as soon as they discover them," Woods said.

"This investigation has revealed important weaknesses in the drug safety legislation in force at the time."

The agency advised doctors in 2003 not to give Seroxat to patients under the age of 18, after Glaxo handed over data showing it could increase their risk of suicidal behavior.

An investigation with a view to criminal prosecution was launched due to concerns that Europe's biggest drugmaker had held the information for some time before this.

In the event, government prosecutors concluded there was no realistic prospect of a conviction and that the case should not proceed to criminal prosecution.