London - Former England striker Alan Shearer has warned that players' lives could be at risk unless urgent action is taken to prevent pitch invaders as the Professional Footballers' Association called for a full inquiry.
Aston Villa captain Jack Grealish was punched by a Birmingham fan and Manchester United defender Chris Smalling was confronted on the pitch by an Arsenal supporter in two separate incidents on Sunday.
It followed an episode on Friday in which Rangers captain James Tavernier was challenged by a pitch-invading Hibernian fan in his side's stormy Scottish Premiership draw.
English football has been blighted by a number of incidents of racist chanting and abuse over recent months that, together with the pitch invasions, are an unwelcome reminder of the hooliganism that used to blight the game.
"It is absolutely disgusting and if we don't stamp it out now, next time it could be a knife," Shearer said in his column in the Sun newspaper.
"It's that serious. Where are we at in football when some thug thinks that is what he is going to do?"
Grealish was assaulted at St Andrew's by Birmingham fan Paul Mitchell, 27, who was later charged with invading the pitch and assault, while Smalling was confronted in United's 2-0 defeat to Arsenal at the Emirates Stadium.
"The football authorities have to be seen to nail Birmingham for the behaviour of their fan," Shearer said.
"If that means docking them points, playing in an empty stadium, so be it. There cannot be a strong enough punishment.
"It's completely ridiculous -- I can't remember seeing anything as blatant as this."
Birmingham said Mitchell would be banned for life from St Andrew's and Arsenal later pledged the same punishment would be imposed on the pitch invader at their match.
United defender Smalling took to Twitter on Monday.
He said: "Commenting would only add to the attention he craved.... Disappointed with yesterday's result but we'll learn from it. Looking ahead. Have a good start to the week everyone!"
Former Aston Villa midfielder Lee Hendrie said the incident at St Andrew's was the worst of its kind that he had seen and backed Shearer's calls for urgent action.
Hendrie told BBC Radio: "It could have been someone running on with a knife -- how are you meant to stop this?
"It's going to take something, that's going to cause a big, big scene in football, that's going to make people really stand up."
Professional Footballers' Association chief executive Gordon Taylor said the attack on boyhood Villa fan Grealish, who scored the winning goal against Birmingham, was "disgraceful" and "cowardly".
He has called on the Football Association to work with the government, police and stewards at the grounds to help stamp out such incidents.
"It was disgraceful, cowardly and criminal. We have seen an increase in such behaviour at clubs this season. It's been racist abuse, it's been anti-Semitism and now it's real fear for the safety of players," Taylor told Sky Sports News.
When asked about what sanctions could be implemented, Taylor said: "There's a combination you can consider -- fines, ground closures, playing games behind closed doors and points deductions.
"Every case needs to be looked at on its merits but it has to be a job for the national association."
Sports Minister Mims Davies said: "The incidents that happened over the weekend were a disgrace.
"It is right that they are investigated immediately and strong action is taken by the football authorities and clubs.
"Protecting players' safety is of paramount importance and every possible step must be taken to ensure that they are not put at risk."