Ambulance chiefs are struggling to meet their target for improving the response times for 999 calls in London.

Almost one in three of emergency calls fail to meet the 14-minute Patient's Charter standard laid down by the Government.

Martin Gorham, London Ambulance Service's chief executive, wants 80 per cent answered within 14 minutes by September. But the figures for May show only a slight improvement: 66 per cent of ambulances arrived on time compared with a Patient's Charter standard requiring 95 per cent, a slight improvement on April (65) andon March's (63 ).

However Unison, the crews' union, says there has been barely any improvement since the immediate aftermath of the computer failure in October 1992. In April 1993, 68 per cent of calls were on time and in May, 67 per cent.

The service refuses to give a breakdown of response times by individual stations, but many are failing to reach 60 per cent.

Unison says the root of the problem is a shortage of crews and ambulances. It alleges that in many areas up to 25 per cent of ambulances are off the road at a time because of cutbacks in overtime and breakdowns.

In July, Mr Gorham will be questioned by the Commons Select Committee on Health. Service sources say that he is desperate to show that there have been real improvements since he took over in 1992.

To try to get response times up, Mr Gorham is proposing to site ambulances on 'active standby, parking vehicles at points of demand such as major roundabouts.

A service spokesman shrugged off the latest figures . 'We are still confident of reaching our target of 80 per cent by September.