LONDON, June 16 -- Both sides in Britain's referendum battle over EU membership halted their campaign Thursday afternoon after Labour Party member of parliament (MP) Jo Cox died after being gunned down and stabbed.

The 41-year-old mother of two was attacked in broad daylight outside a public library. The Yorkshire Post newspaper reported she was attacked by a man reportedly shouting "Britain first." Police in the county said a 52-year-old man had been arrested in connection with the murder.

The MP, who represents Batley and Spen in one of the industrial heartlands of Yorkshire, was in a critical condition but desperate attempts by doctors to save her were in vain. British Prime Minister David Cameron and Labour leader Jeremy Corbyn led tributes to the slain politician.

Corbyn said: "Jo Cox died doing her job at the heart of our democracy, listening to and serving the people she was elected to serve. The whole country is in shock and grief at the horrific murder."

With just a week to go before 43 million Britons decide whether Britain's future is to stay in or leave the 28-member European Union (EU), Thursday had been slated as a busy day to win over the millions still undecided.

Earlier, the leaders of Britain's 10 biggest provincial cities joined forces Thursday to warn that leaving the EU would pose a grave threat to local economies.

The Core Cities Group includes Britain's biggest provincial city, Birmingham, as well as Glasgow and the Welsh capital of Cardiff.

Many British cities have been the recipients of Brussels funding through European regional development funds.

Leaders of the 10 cities said in a joint statement: "Leaving would be a grave threat to our local economies, risking people's jobs and livelihoods. If we vote for Brexit, it will be those at the sharp end -- working people -- not the leaders of the leave campaign, who will pay the price. A vote to leave is a vote for serious economic danger. It is simply not worth the risk."

They said membership of the EU had created 63,000 jobs and protected another 16,800 across the 10 city regions, and provided 2.54 billion U.S. dollars of investment to help grow urban economies.

Also signing the statement were the leaders of Bristol, Liverpool, Leeds, Manchester, Newcastle, Nottingham and Sheffield.

The focus of the campaign had been the north of England, the traditional Labour stronghold area where the canvassing of party members showed strong support for leaving the EU.

Labour's Jeremy Corbyn on a visit to the north said Thursday many Labour voters backing Brexit "haven't understood" the benefits of the EU.

A vote to leave could also hit global markets, according to a warning issued Thursday by the Bank of England's Monetary Policy Committee (MPC).

In its statement, issued Thursday, the bank said a vote to leave the EU could materially alter the outlook for output and inflation, and therefore the appropriate setting of monetary policy.

The latest IPSOS-Mori poll puts the "leave" voters at a six-point lead, despite the most intensive campaigning by the "remain" camp over the past few days.