A shop assistant was in a critical condition in hospital today after being stabbed in the back as he tried to stop several youths thought to have been shoplifting.

The 19-year-old man was attacked yesterday afternoon in a service area at Wood Green shopping centre, north London after an altercation with three men. Detectives are examing CCTV footage of the area.

The attack comes as ministers consider bringing in tougher sentences for knife possession amid growing public concern at the increase in attacks.

Vernon Coaker and Lord Bassam of Brighton, Home Office ministers, are examining the possibility of automatic jail terms for carrying knives, as with the unauthorised possession of guns.

Police are investigating whether the teenage victim, who is of Mediterranean appearance but has not yet been named, knew any of his attackers and whether they worked at the shopping centre.

A Scotland Yard spokesman said: "It appears there was an altercation between the victim and three men, during which he was stabbed.

"The shopping centre closed after the assault, with staff and shoppers requested to leave."

Police, who have yet to make any arrests, are investigating whether the victim knew one of his attackers and whether any of them worked at the shopping centre.

Michael Thompson, general manager of the shopping centre, said security would be reviewed following the attack.

He said: "There was an altercation between a shop assistant and some individuals who we believe may have been involved in shoplifting.

"The altercation took place on the first floor and took place in a service corridor.

He added that some of the shops had their own security staff and the shopping centre also provided security personnel.

Earlier this week a 29-year-old father was stabbed in Bristol in front of his 3-year-old daughter in an argument over the price of a pint.

The victim, named by neighbours as Barry Wilson, had been drinking with his attacker. The stabbing happened after a police-sponsored street clean-up event. Mr Wilson is believed to have helped residents on his estate to collect 42 skiploads of rubbish. Friends said that an argument erupted when the man asked for 40p towards the cost of another pint and Mr Wilson refused.

Paul Burgess, a neighbour on the Knowle West estate, said: "I saw Barry and the other chap fighting in the street for about five minutes. They were going in and out of each other’s houses." Mr Burgess said: "Barry came out with a hammer and this guy must have had a knife hidden up his sleeve. He stabbed him in the neck and then again as he fell to the floor. There were about 20 or 30 neighbours out watching it."

The catalogue of recent knife attacks includes an attack on Ian Montgomery, who was critically injured after going to the aid of a woman being attacked. Thomas Grant, a student, was killed on a train at the weekend after intervening in a dispute, and Marlvin Jiro was stabbed to death in a fight after a night out in Birmingham.

In London, the head teacher of the school attended by Kiyan Prince, the murdered 15-year-old boy, added his voice to the chorus of those calling for a stronger stance on knife crime. Phil Hearne, principal of the London Academy in North London, said: "We are happy to tackle unhealthy eating in schools, but which is the more dangerous — a turkey twizzler or a knife?"

IF ATTACKED . . .

Flee. If you cannot, shout "fire" to draw the attention of passers-by

Be submissive and passive, not confrontational. Keep eye contact with attacker

Try to keep space between you and the knifeman, possibly with a bollard or a lamppost

If you are attacked, try to minimise the area of attack and protect the vital organs. Cover heart

Make yourself as small as possible. Fall to the ground and make yourself into ball to keep the target area as small as possible

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