The prime minister promised to deliver free scrips for people with long-term conditions at Labour's conference in September 2008.
Healthcare Republic exclusively revealed in December that charities feared the impending election meant there would not be enough time to legislate for the pledge.
Prescriptions are already free in Wales and charges will soon be abolished in Scotland and Northern Ireland.
Speaking on behalf of the Prescription Charges Coalition, Neil Churchill, chief executive of Asthma UK, said: ‘People with long-term conditions will not accept this policy falling through the cracks. Gordon Brown must listen to today's wake-up call and deliver on his prescription promise without delay.
‘People across the country are being forced into a position where they stretch their medicines or stop taking them altogether because of the cost, which can lead to worsening health and even result in hospital admissions. This is an unacceptable state of affairs, so the message is, Gordon, time is running out - act now before it's too late.'