Accident and emergency (A&E) waiting times at Kingston Hospital rose only slightly last year despite spending cuts, figures show.
The department has only missed its monthly target of treating 95 per cent of patients within four hours once in the last two years.
Hospitals across the country have to save £20bn by 2015.
A&E staff see about 8,700 people every month – nearly 300 a day – and 2,500 more people visited the department in 2012 compared with 2011.
The proportion of patients seen within four hours dropped from 97 to 96 per cent in the same period.
Michael Walker, Unison regional officer in south-west London, said: "We are always sceptical about targets, however anecdotal evidence is that they have never worked so hard trying to get patients seen. They are under immense strain. That is not sustainable in the long run."
He said changes such as the introduction of NHS 111, a new phone line for people needing urgent treatment, and the Better Services Better Value (BSBV) review, in which two A&E departments in south-west London will close, will cause patient numbers to soar.
He said: “We would ask where the extra resources are coming from to pay for the extra nurses that will be needed in A&E.
“We are fearful that when NHS 111 comes in in April there will be a massive spike of people coming in.”
It was announced on Tuesday that following pressure from councils, a decision by BSBV board members on which hospitals in the area will lose their front-line services has been postponed.
Epsom and Ewell MP Chris Grayling welcomed the move.
And about 150 campaigners marched from Norbiton station to Kingston Guildhall on Saturday to protest cuts to the health service.
Nora Pearce, Unison representative at Kingston Hospital, was at the march and said the welcome treatment figures could drop as a result of the BSBV review.
She said: “It is the staff on the ground that save everybody because they work so hard.”