Influential backbench Conservative Andrew Tyrie will put his weight behind a Remain vote with just 10 days to go until the referendum.
Tyrie, who chairs the Treasury Select Committee, is to say that leaving the EU will almost definitely bring about a "short-term economic shock", which would hamper economic growth and bring down living standards.
He will say that the strongest economic argument is to Remain, though there are reasonable arguments on both sides.
"On the economic impact, most experts – among them Brexit supporters – have concluded that a short-term economic shock would almost certainly accompany leaving the EU," the former Treasury advisor will say.
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"This would reduce the growth of GDP below the level it would otherwise achieve, probably bringing lower living standards.
"Most experts have concluded that there will also be a long run cost, although this is much less certain. There are economic risks to staying and leaving; but the risks are greater to leaving.
"Over the longer term, it is reasonable to conclude that Brexit would lead to a loss of GDP because leaving the EU would result in less access to the single market. Behind the cacophony of claim and counter-claim lies this irreducible point.
"In this referendum, the Leavers have the simplest points. But the Remainers have the better of the argument."
While Tyrie's pro-EU views have been known, his decision will be thought of as a big win for the Remain campaign given his clout.
The committee which he chairs recently published a report on the costs and benefits of EU membership, raising concerns over statements by both Remain and Leave.
On the Leave side, the report sounded the alarm over the "highly misleading" claim that the UK contributes £350m a week to the EU.
Tyrie will add: "This is nonsense politics. It is a form of electoral bribery. It is of an order of magnitude worse than usually encountered in general elections – £350m a week or £50m a day, is a false prospectus.
"It has some of the corrosive characteristics of Tony Blair's claims on Iraq. I very much regret that the electorate has been expected to wade through this mountain of nonsense to find grains of truth."
Meanwhile, it said that the Treasury had not fully explained how it reached a figure of £4,300 as the cost to households by 2030 if the UK votes for Brexit.