Oliver Cromwell is often held up as the embodiment of strength and power.

He overthrew and beheaded King Charles I in the English Civil War, waged a bloody campaign against Catholics in Ireland and is the only commoner to have become head of state in British history.

So impenetrable, contemporaries dubbed him “Old Ironsides”. So brutal, historians have called him a “regicidal dictator”.

But a letter being made public for the first time shows that he was plagued with depression and asked friends to pray for him, as his mental health suffered severely at a time when he was arguably at his most powerful.

The letter, written to an unknown friend in 1652 “will seem familiar to anyone who suffers or has been diagnosed with depression,” said Stuart Orme, curator at the Cromwell Museum in Huntingdon, Cambridgeshire where it will go on display.