Dave Prentis, leader of Unison, said downgrading the services will have long-term consequences for the capital.
The union accused the Government of failing to listen to the concerns and arguments of clinicians, patients, hospital staff, unions and the local community.
The campaign to protect Lewisham's A&E and maternity services has shown that people are willing to stand up and fight to keep their NHS safe and local.
The long-term impact of the downgrade - and whether today's announcement is just a repackaging of the original plans - remains to be seen.
We know that a major reorganisation of emergency services is planned, which makes today's decision even more premature.
Lewisham is not the first and won't be the last hospital to come under attack by this Government. Its disastrous NHS reorganisation and squeezes on funding are causing cuts and closures across the country."
The Government was accused of "breaking up" the NHS after announcing a downgrading of Lewisham A&E and maternity services
An A&E department and maternity services at a major hospital will be downgraded, Health Secretary Jeremy Hunt told MPs.
The proposed closure of units at Lewisham Hospital is part of a radical plan to rescue a neighbouring NHS trust from bankruptcy.