The Former Prime Minister John Major is set to issue a thinly veiled attack on Boris Johnson and Michael Gove, warning that Tories leading the campaign to leave the EU risk “morphing into Ukip”.

In a speech on Friday Sir John will say that as the economic arguments in favour of Brexit get demolished so Tory Leave campaigners have increasingly returned to the theme of immigration.

This, he will say, is straying into “dangerous territory” and risks “opening up long-term divisions in our society”.

“As the Leave arguments implode one by one, some of the Brexit leaders morph into UKIP, and turn to their default position: immigration,” he said.

“This is their trump card. I urge them to take care: this is dangerous territory that – if handled carelessly – can open up long-term divisions in our society.

What to believe about the EU referendum

“Of course, it is legitimate to raise the issue of the sheer number of those wishing to enter our country. I wholly accept that. Nor do I wish to silence debate. We mustn’t overlook genuine concerns: but these should be expressed with care, honesty and balance. Not in a manner that can raise fears or fuel prejudice. The leave campaign are crossing that boundary, and I caution them not to do so.”

His intervention will infuriate many Eurosceptic Tories, who believe that the Leave campaign should focus on immigration ahead as the best way of winning the June 23 vote.

What's the European Parliament ever done for us? Show all 5 1 /5 What's the European Parliament ever done for us? What's the European Parliament ever done for us? A cap on the amount of hours an employer can make you work The Working Time directive provides legal standards to ensure the health and safety of employees in Europe. Among the many rules are a working week of a maximum 48 hours, including overtime, a daily rest period of 11 hours in every 24, a break if a person works for six hours or more, and one day off in every seven. It also includes provisions for paid annual leave of at least four weeks every year Getty Images What's the European Parliament ever done for us? Helping the people of Britain to avoid smoking In 2014 MEPs passed the Tobacco Products Directive strengthening existing rules on the manufacture, production and presentation of tobacco products. This includes things like reduced branding, restrictions on products containing flavoured tobacco, health warnings on cigarette packets and provisions for e-cigarettes to ensure they are safe What's the European Parliament ever done for us? Helping you to make the right choices with your food Thanks to the European Parliament, UK consumers have access to more information than ever about their food and drink. This includes amount of fat, and how much of it is saturated, carbohydrates, sugars, protein and so on. It also includes portion sizes and guideline daily amount information so people can make informed choices about their diet. All facts must be clear and easy to understand What's the European Parliament ever done for us? Two year guarantees and 14-day returns policy for all products Consumers across the EU have access to a number of rights, from things which are potentially very useful, to things which used to be annoying. For example, shoppers in the UK receive a two-year guarantee on all products, and a 14-day period to change their minds and return a purchase, these things are useful www.PeopleImages.com-licence restrictions apply What's the European Parliament ever done for us? Keeping your air nice and fresh (and safe) Believe it or not, although the situation is improving, some areas of the UK have appalling air quality. A report by the Royal College of Physicians released on 23 February says 40,000 deaths are caused by outdoor air pollution in the UK every year. Air pollution is linked to a number of illnesses and conditions, from Asthma to diabetes and dementia. The report estimates the costs to British business and the health service add up to £20 billion every year

But Sir John, whose on leadership was dogged by arguments with his Eurosceptic backbenchers, will add: “We must not let emotions be stirred by false fear: nor allow that false fear to impair our judgement on the future of our country.