Campaigning for next week's EU referendum in Britain was suspended Thursday after a pro-European lawmaker was reportedly shot and injured in her constituency in northern England.

Jo Cox, 41, a mother-of-two who represents the opposition Labour party, was left bleeding on the pavement after the incident in Birstall in Yorkshire, the Press Association cited an eyewitness as saying.

Police said a woman in her 40s was in a critical condition after suffering serious injuries but did not confirm her name, adding 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".

"Very concerned about reports Jo Cox has been injured. Our thoughts and prayers are with Jo and her family," Prime Minister David Cameron added.

Channel 4 News quoted Cox's office as saying that the MP had been shot and stabbed. The BBC said she was flown by air ambulance to a hospital in nearby Leeds.

Cox, who was elected for the first time last year, was a supporter of Britain staying in the EU in the referendum on June 23, although it was not clear if the attack had anything to do with the campaign.

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

Both sides of the referendum race announced they would stop campaigning for the rest of Thursday.

"We are suspending all campaigning for the day. Our thoughts are with Jo Cox and her family," the official Stronger In Europe campaign said on Twitter.

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

"The battle bus that Boris was on is turning back to London," he told AFP, referring to lead campaigner Boris Johnson, the Conservative former mayor of London.

The Daily Telegraph reported Johnson as describing news of the attack as "absolutely horrific".

- 'Critical condition' -
"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 force said.

Contacted by AFP, her offices in London and Yorkshire could not immediately be reached.

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

"Utterly awful news about an attack on a superb and wonderful colleague Jo Cox -- hoping and praying for her and her family right now," fellow Labour MP Chuka Umunna said.

MPs in Britain do not usually have any security staff and attacks on lawmakers are rare.

Labour MP Stephen Timms was stabbed twice in the stomach while meeting with constituents in 2010.

A 21-year-old student was later jailed for life for trying to kill the MP because he voted for Britain's 2003 invasion of Iraq.