Former British prime ministers John Major and Tony Blair will be in Northern Ireland today
A leading Brexit supporter in the UK has defected to the 'Remain' camp, saying she cannot support a campaign she 'knows to be untrue'.
Switching sides, Conservative MP Sarah Wollaston hit out at Vote Leave's claim that leaving the EU would give the UK's National Health Service an extra £350m (€445m) a week.
"For someone like me who has long campaigned for open and honest data in public life I could not have set foot on a battle bus that has at the heart of its campaign a figure that I know to be untrue," she told the BBC.
Dr Wollaston later tweeted that she believes the NHS will in fact be "safer" if the UK votes to remain.
"Having listened carefully to both sides of the debate, I believe our NHS & research will be safer if Britain remains in the EU," she said.
"If I woke on the 24th June to the news that Britain had voted to leave the EU, I would feel a sense of loss rather than freedom. Of course I understand that politicians are not allowed to change their minds but real people do."
Hailing the move, prime minister David Cameron said she had made a "powerful intervention".
A Vote Leave spokesman described her decision as "bizarre" and added: "Given her views on the EU in the past it is disappointing to suddenly see her repeating lines straight from the Remain campaign hymn sheet."
The Leave campaign has also been pulled up on its initial claim the £350m would go to the NHS and have revised it down more recently to say £100m (€127m) would be spent on the NHS.
Meanwhile, former prime ministers John Major and Tony Blair will be in Northern Ireland today, campaigning to keep Britain in the EU.
Eligible Britons now have until midnight on Thursday to sign up after many were left disappointed by technical problems during a last-minute rush to register late on Tuesday evening.
Mr Cameron's official spokeswoman has said the extension is "legally watertight" and confirmed the Government had held talks with the Electoral Commission and opposition parties.
However, some critics have accused the prime minister of attempting to skew the referendum result by giving young people, who are statistically likelier to support the In campaign, extra time to register.