THERESA May’s decision to put up Boris Johnson to give the first of her Cabinet’s “Road to Brexit” speeches designed to woo Remainers backfired yesterday after the gaffe prone Foreign Secretary was plunged into renewed controversy after making an offensive joke about sex tourism to Thailand – which has notoriously involved the abuse of children.

His address, meant to be a serious attempt to unite people in the UK ahead of its departure from the EU, descended into the bizarre as he compared the Prime Minister to the Old Testament’s Moses, described the English Channel as a moat, talked about Britons going on stag parties to European capitals and said Brexit would be good for “organic carrots”.

But it was his remarks on British tourists going to Thailand for sex which prompted the most criticism.

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The written copy of his speech issued to journalists beforehand said: “As I have just discovered we have more than a million who go to Thailand every year, where according to our superb consular services they get up to the most eye-popping things.”

His actual speech when delivered toned down the "joke" slightly – "where our superb consular services deal with some of the things they get up to there" – but the comments still prompted new calls for his sacking.

James Dornan, the SNP MSP, who is the convener of Holyrood’s education committee, said: “Once again Boris Johnson shows he is not fit to hold the role of MP never mind Foreign Secretary.

“His comments about sex tourism are utterly appalling and Theresa May should consider his position as Foreign Secretary and he should consider his position as an MP.”

Dornan added: “As long as Scotland is part of the Uk this man represents us and I do not want someone with his views to be speaking on my behalf. The first thing he should do is apologise, and then he should go away and think about his future.”

The sex tourism comments were also seized about by people on social media.

Scottish journalist Liam Kirkcaldy tweeted: “In a keynote speech on Brexit Boris Johnson is talking about measuring the English channel with his fingers, referring to it as a moat, and making jokes about the things that go on when Brits go to Thailand. This is terrifying.”

Another Twitter user said: “Boris Johnson going on about Brits committing illegal activities in Thailand and stag parties in ancient cities. This is our international reputation? How sad.”

A third wrote: “Harming our reputation and making light of serious behaviour, even criminal abuse, is a step too far even for him.”

It is the latest gaffe by Johnson who has been stung by a series of controversies during his career as Foreign Secretary.

Last year calls for his sacking were made after he suggested the Libyan city of Sirte might become a new Dubai once “the dead bodies” are removed.

Other remarks saw him during a select committee in Westminster incorrectly say a British mother – currently detained in Iran – was training journalists in the region. After his comments, 38-year-old Nazanin Zaghari-Ratcliffe was hauled in front of an Iranian court and told her sentence may be doubled to ten years.

And during a visit to India in January last year he appeared to liken Francois Hollande, the former French President, to a Second World War German general and wanting to administer World War Two-style “punishment beatings”.

Thailand has a notorious problem with prostitution and the sexual abuse of children by western tourists. Efforts are being made by the authorities to crack down on the crime and last month 80 women and girls were rescued at one massage parlour in Bangkok. The raid followed a tip-off from a 14-year-old who said she had sold sex there after being trafficked from Myanmar when she was 12-years-old.

During his speech Johnson insisted on the need to diverge from EU rules after leaving the EU and repeated his view the UK should not have to follow these regulations during a transition period.

In an effort to address concerns raised about the hit to the economy, he said: “To those who worry about coming out of the customs union or the single market – please bear in mind that the economic benefits of membership are nothing like as conspicuous or irrefutable as is sometimes claimed.” The Brexit-backing minister, who said during the referendum campaign that leaving the EU would mean an extra £350m a week to the NHS, went on to claim that Brexit could be “grounds for much more hope than fear”.

The SNP’s Europe spokesman Stephen Gethins hit back: “Boris Johnson is literally the last person who the Tories should wheel out to try and reassure worried Remain voters about Brexit. Instead of peddling his embarrassing hard Brexit fantasy he should be apologising for the notorious £350m-a-week-for-the-NHS deception for which he was responsible ... Anyone hoping for answers and reassurance from the gaffe-prone Foreign Secretary today would have been sorely disappointed.”

The speech is the first of six being made by Prime Minister Theresa May and senior Cabinet figures to set out the Government’s road map for its Brexit negotiations.