GUN crime in Britain will escalate sharply as drug gangs battle for supremacy, police experts will warn ministers after the election. Police estimate that nearly 300,000 illegal guns or replicas capable of being reactivated are now in circulation.

The number of firearm offences increased from 4,903 in 1997 to 6,843 last year, but ministers will be told that a particularly high surge in murders using firearms in the past 18 months will continue, with more and more criminals prepared to use weapons to defend their businesses and territory.

Trafficking in the weapons is also on the increase, the National Criminal Intelligence Service (NCIS) will tell the prime minister in July, when it issues its confidential threat assessment.

The sharp warnings will be accompanied by new pleas from senior police officers for stricter gun laws. David McCrone, deputy chief constable of Greater Manchester police and the UK's most senior police officer dealing with firearms, does not rule out the possibility of growing numbers of armed police on the beat if the situation does not change.

In recent weeks two men have been machine-gunned to death in a residential street in north London, a pensioner was hit by a stray bullet during a shoot-out in Manchester and, in the past 18 months, three "drive-by" shootings have alarmed police in both cities.

Last Monday, Alice Carroll, 70, was shot in the back as a gunman opened fire on another man near her home in a quiet residential cul-de-sac in Longsight, Manchester. The divisional police headquarters is just 200 yards from her door. She is the first innocent bystander to have been caught up in the new wave of drug violence sweeping the city. Police think she may have been shot when a meeting between drug dealers turned nasty.

Carroll said: "We had a few people trying to sell drugs a couple of years ago but a new woman officer took over in the area and cleared them all out. I was just in the wrong place at the wrong time."

Greater Manchester police say that in the 12 months to March last year there were eight deaths by shooting in the force's area, and nine the following year.

Some forces are now understood to be considering increasing the number of officers who are armed routinely. Last year Nottinghamshire became the first force to arm beat officers with handguns in a trial project.

McCrone, who sits on the government's firearms consultative committee (FCC) and is head of the Association of Chief Police Officers' (Acpo) committee on firearms, says: "I don't think armed British police are inevitable, but we need to do more to make sure that doesn't happen.

"Many criminals are now 'renting' guns for killings. We see the same weapon being used in different parts of the country in different scenarios."

He will demand new laws to outlaw many replica guns and to ban the license-free ownership of machines to reload bullets. Home-loaded rounds are most commonly used in gangland killings as legal or stolen rounds are less readily available since the ban on handguns was introduced in the wake of the Dunblane shootings in 1996.

NCIS will warn in July that there remains a "strong link" between firearm possession and drug trafficking. It will say that many young men carry weapons as a "fashion accessory" but use them in violent situations in the way that they might previously have used a knife. The largest number of guns is imported from western Europe, although more unusual weapons, such as rocket launchers, have been imported from the former eastern bloc.

According to police, the price of contract killings has fallen from thousands of pounds to as little as £200, with the killers now more likely to be drug addicts than professional assassins. A north London crime family pioneered the idea of using crack addicts to kill its rivals because they are seen as "disposable" and their evidence in court is often unreliable.

The rise in gun crime has mirrored sharp overall rises in violent crime. While Jack Straw, the home secretary, was able to bring about a 0.2% fall in crime overall, violent crime has more than doubled in the past four years, although part of this has been attributed to a change in statistical methods. Street robberies, muggings and attacks have grown by 30% in some places, while drink-related crime has also risen sharply.