A British lawmaker was critically ill on Thursday after reportedly being shot in her constituency in northern England, in a broad daylight attack that caused campaigning for next week's EU referendum to be suspended.

Jo Cox, 41, a mother-of-two who represents the opposition Labour party, was shot three times and also stabbed by a man in his 50s in the village of Birstall in Yorkshire, a witness told the BBC.

Another witness, Hichem Ben Abdallah, told the Press Association she had been shot twice.

Police confirmed that a woman in her 40s was in a "critical condition" but by convention did not name her. A spokesman said that a 52-year-old man had been arrested.

Labour leader Jeremy Corbyn said on Twitter that he was "utterly shocked by the news of the attack on Jo Cox. The thoughts of the whole Labour party are with her and her family at this time".

Clarke Rothwell, a cafe owner in Birstall, told the BBC that he was alerted to the incident by a "loud popping noise that sounded like a balloon burst -- a loud balloon".

"When I looked round there's a man stood there in his 50s with a white baseball cap on and a jacket with a gun, an old-fashioned looking gun in his hand," he said.

"He shot this lady once and then he shot her again, he fell to the floor, leant over shot her once more in the face area.

"Somebody tried to grab him, wrestling with him and then he wielded a knife, like a hunting knife, just started lunging at her with a knife half a dozen times.

"People were screaming and running from the area."

Channel 4 News quoted Cox's office as saying that the MP had been shot and stabbed. Her staff could not be immediately reached when contacted by AFP.

The BBC said she was flown by air ambulance to a nearby hospital.

Prime Minister David Cameron said he was "very concerned" about the attack and cancelled a rally he had planned in Gibraltar as part of his campaign for Britain to stay in the EU on June 23.

Cox is in favour of the "Remain" campaign, which also suspended all other campaigning Thursday, although it was unclear whether the attack was related to the referendum.

Sky News television quoted unconfirmed reports from witnesses that Cox's shooter shouted "Britain first" -- possibly a reference to British sovereignty, although there is also a far-right party called Britain First.

A spokesman for the rival Vote Leave group, which is backing a so-called Brexit, said that its 'battle bus' had stopped campaigning for the day.

European Commission president Jean-Claude Juncker tweeted his condolences, saying he was "deeply shocked".

- 'What is happening?' -
"At 12.53 today, police were called to a report of an incident on Market Street, Birstall, where a woman in her 40s had suffered serious injuries and is in a critical condition," West Yorkshire police said in a statement.

"A man in his late 40s to early 50s nearby also suffered slight injuries.

"Armed officers attended and a 52-year-old man was arrested in the area."

The attack took place near the town's library where she regularly held meetings with constituents.

Fellow MPs took to twitter to express their horror at the attack, with Labour MP Andy Burnham writing: "What is happening to the world?"

Most British MPs have little or no security but such incidents are extremely rare.

Labour MP Stephen Timms was stabbed twice in the stomach by a 21-year-old student in 2010 because he voted for Britain's 2003 invasion of Iraq. His attacker was jailed for life for trying to kill him.