A police firearms officer has said a budget reduction could lead to fewer armed patrols on London's streets.

The Metropolitan Police said it would cut £680,000 from CO19, the force's specialist firearms unit.

It comes after the force said it would have armed patrols in south London following a spate of shootings in which three teenagers died.

Pc Mark Williams said the cuts would affect those patrols. The force said cuts would not influence core policing.

A spokeswoman said it would make the savings by reorganising six police officer posts at its firearms training school and reducing overtime payments.

She said the posts at the training school would now be filled by staff - normally retired police officers - working as firearms instructors.

"These members of police staff, all of whom are former police officers will work alongside police officer firearms instructors at the Met's Firearms Training Centre," she said.

"The CO19 senior management team will ensure that future firearms training benefits from the right mix of operationally experienced police officers and police staff."

She said further savings would be made by focusing on overtime costs.

"Overtime has always been, and will remain a contingency and should not affect core policing," she said.

But Pc Williams told Radio 4's Today programme: "We're flabbergasted.

"Cuts in overtime will always have an effect.

"A lot of the stuff we do with overtime is intelligence led and responding to events to put armed response units out, such as recently with the very sad shootings of young people in south London.

"The public in London must know we will do our utmost to make sure we will have adequate armed response cover.

"But inevitably if you have cutbacks of the size the are talking about, it will have an effect on many things we do."

On Thursday, the Home Office figures revealed the number of times the Metropolitan Police deployed guns had increased by more than 60% in a year.

In 2005, guns were taken out on 2,964 occasions. That rose to 4,711 in 2006.

The force had 1,823 authorised firearms officers in 2002 - by last year this increased to 2,332.