Campaigning for the EU referendum officially gets underway today (15 April) as both sides prepare to bring out the heavy hitters. The campaign period will last 10 weeks and conclude on polling day on 23 June.

Spending limits have been set between £700,000 ($990,900) and £7m – the latter for the official campaigns, Stronger In and Vote Leave. London Mayor Boris Johnson and Justice Secretary Michael Gove have both thrown their support behind the leave camp and will urge for taxpayer money to be spent on the NHS rather than the EU. The pair's calls will be supported by Vote Leave chairwoman and Labour MP, Gisela Stuart.

"The NHS is facing a crisis due to growing demand for healthcare and a squeeze on funding. This means that patients are not always getting the treatment that they need, when they need it," she said. "If we vote leave we will be able to stop handing over £350 million 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.

"If we vote leave the money we send to the EU will plug some of the gaps in the NHS funding crisis – that's why leaving is the safer choice in this referendum."

But these claims have been slammed by former NHS England chief executive, Sir David Nicholson, who said the assertions were "untrue" and that a Brexit would do more harm than good. "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," he said.

Meanwhile, former Labour chancellor Alistair Darling will make the economic case for Britain staying in the EU. He will make a speech criticising Vote Leave for "playing with fire and asking the British people to play along".

Darling, who served as chancellor from 2007 to 2010, will accuse Vote Leave of encouraging the electorate to take a leap into the unknown. "They have no idea how long it would take to negotiate a deal, or how much damage the uncertainty would do to our economy," he will say. "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."