The official campaigning ahead of the EU referendum in Britain begins later, with both sides bringing out their heavyweights for a day of speeches and rallies.

London Mayor Boris Johnson will claim Britain's contribution to the EU budget could instead be spent on their health service, while former chancellor Alistair Darling will warn that Brexit could wipe almost stg£100bn off (€125bn) UK trade.

In language that will infuriate British Prime Minister David Cameron, Vote Leave - which is jointly led by UK Justice Secretary Michael Gove - will also accuse the government of failing to give the health service "the funding it needs".

Vote Leave chairwoman and Labour MP Gisela Stuart will add her voice to the argument, saying: "If we vote Leave we will be able to stop handing over £350m a week to Brussels and we will be able to instead spend our money on our priorities like the NHS. This will improve patient care".

However, the Stronger In campaign accuses Vote Leave of using incorrect figures on the UK's contribution to the EU budget.

A spokesman said: "They claim our net contribution is over £10bn when the Institute for Fiscal Studies has shown it to be half this at £5.7bn, which amounts to 24p per person per day".

"Untrue" claims on the NHS

Former National Health Service (NHS) England chief executive David Nicholson also said Vote Leave's claims about the NHS were "untrue".

He said: "Leaving the European Union would damage the economy and that would mean less money for the NHS - not more - as well as destroying the countless benefits of membership for health priorities like science and research"

The economic argument for Britain remaining in Europe will be set out by Mr Darling, who will accuse Vote Leave of "playing with fire and asking the British people to play along".

He will point to new research, which suggests that if the UK were to follow the Canadian free trade agreement with the EU, trade could fall by stg£92bn (€115bn).

Mr Darling will say: "They have no idea how long it would take to negotiate a deal, or how much damage the uncertainty would do to our economy".

"They are offering a fantasy future where we keep all the benefits of being in Europe without being part of the single market. It's Project Fantasy".

The referendum will be held on June 23rd.

It comes as a new campaign is asking British people living in Ireland to vote in the referendum.