RUTH Davidson failed to defend Boris Johnson over his suggestion that the Leave campaign was guilty of a “gross underestimate” when it said Brexit would mean an extra £350 million a week for the NHS.

In a television interview last night, the Scottish Tory leader was asked about remarks made by Johnson in a newspaper interview.

The Foreign Secretary told the Guardian the UK is contributing £362m a week to the EU and that figure would rise to £438m by the end of the post-Brexit transition period.

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“We grossly underestimated the sum over which we would be able to take back control,” he said.

Appearing on ITV Border last night, Davidson was asked about Johnson’s comments. Journalist Peter MacMahon reminded her of the debate she had with Johnson at London’s Wembley Arena during the EU referendum campaign, in which Davidson accused Johnson of lying over the £350m a week figure.

MacMahon asked Davidson if Johnson was lying again. She replied: “Well, look, he’s a member of the Cabinet and he gets to see the Treasury’s books.

“I’m not a member of Government and I don’t. But my understanding from many economists and government watchers who were there at the time was that I was correct in what I said when I was on the stage at Wembley debating it.”

MacMahon responded: “So he was lying then and he’s lying now?”. Davidson replied diplomatically: “Like I say, I can only take the analysis that I’ve seen, that I’ve seen from economists around the world, and we believed in the arguments that we were putting forward for Remain.”

During the EU referendum campaign, Davidson also launched a scathing attack on Johnson when he appeared on the BBC’s Andrew Marr Show .Responding to his grilling, in which he struggled to make clear his vision of Britain outside the EU, Davidson tweeted: “Is it just me or is Boris floundering here? Not sure the bumble-bluster, kitten smirk, tangent-bombast routine is cutting through.”

After the UK voted to leave the EU, and Scotland voted to remain, Davidson appeared to move over to support the Leave side, and was labelled a “born-again” Brexiteer.

As the differences between the two politicians emerged again, one leading health expert called Johnson’s remarks on the benefits to the health service of Brexit “ridiculous”.

Speaking in London at a conference about reducing health inequalities, Professor Sir Michael Marmot was asked what he made of the Foreign Secretary’s claim.

He replied: “I think when the history of the 21st century is written, Boris Johnson will go down as one of the best guests on Have I Got News For You. It’s a ridiculous thing to say. It’s been pointed out by the statistics authority, the national statistics authority, that that is totally misleading”.

Marmot added: “He does say ‘gross’. Gross is a good word to use actually ... but that’s not the relevant bit.” The academic said the claim has been shown to be “a lie”, adding: “It’s an untruth. It’s misleading. I’m not sure what other word to use.”

Asked about Johnson’s claim the UK could regain control of more than £350m a week after Brexit, Theresa May’s official spokesman said: “Does the amount of money we send to the EU fluctuate year on year? Yes, it does. Some years it is bigger, some years it is smaller.”

Labour has asked the statistics watchdog to comment on Johnson’s claim. Shadow Brexit secretary Sir Keir Starmer has written to Sir David Norgrove, chairman of the UK Statistics Authority, about the Foreign Secretary’s comments.

Starmer said: “The £350m a week claim made by the Vote Leave campaign has been widely condemned as inaccurate and misleading.

“For example, in September of last year, the Statistics Authority wrote to the Foreign Secretary saying ‘it is a clear misuse of official statistics’.”