The Liberal Democrat leader said there had been a “failure in British diplomacy” after the G7 failed to agree sanctions on Russia after their alleged involvement in chemical attacks in Syria.
G7 nations were meeting in the Italian city of Lucca following last week’s chemical attack on the rebel-held Syrian town of Khan Sheikhoun, which left 89 people dead.
The Turkish health ministry on Tuesday confirmed the deadly nerve agent sarin had been used.
The attack was allegedly ordered by Syrian leader Bashar al-Assad, however, his administration denied involvement, leading to the US firing 59 Tomahawk missiles at a government airbase in response.
Tim Farron hit out at Brexit while discussing the Syrian conflict
The United Kingdom spends its time going around the world, particularly Europe, alienating leaders
Mr Farron, a staunch critic of Brexit, accused the Foreign Secretary of “undermining” European colleagues while campaigning to guide Britain out of the EU.
Speaking on BBC News, he said: “It appears there has been a failure in British diplomacy.
“I have to say, all of the evidence is that Boris Johnson, himself and this current Government over the last 12 months seem to have done so much to have to undermine the other countries – with the exception of America – around that table that they’ve not got a hearing.”
Mr Farron argued he suspects Mr Johnson did not ask the correct questions at the negotiating table, by suggesting a “humanitarian corridor” is introduced in Syria.
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The Lib Dem said there will be “no peaceful future” for the region with President Assad in charge, blaming the UK, once again, for not putting up a stern challenge.
In another dig at Brexit, Mr Farron added: “This a reminder that the United Kingdom – and I’m afraid Boris Johnson is probably the greatest offender – spends its time going around the world, particularly Europe, alienating other leaders.
“It cannot be surprised when it fails to take those countries with them on something as vital as this.
“It is a representation of Britain’s reducing role in the world and a reminder while this was a summit for foreign ministers, Theresa May – who I do not begrudge having a holiday – this is such a moment of great importance, she needs to be in London taking control.”
In response to allegations of failure at the negotiating table, Mr Johnson insisted his proposals “had a wide degree of acceptance but you’ve got to do these things in the proper, legal way”.
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Remainers are finding it hard to accept Brexit.
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He told Sky News: “What I think we all want to see is Russia engaging with a political process that involves a transition to a new government in Syria.
“I think that’s the offer that Rex Tillerson is going to be making, and I think it’s a powerful one, but you must be realistic that we’ve been here before.
“Did they know that Assad was going to unleash chemical weapons? We have no evidence of that. It may very well be that they’ve simply been betrayed by their client, by the gun they’ve been backing.
“Now is the time for them to recognise that and help the rest of the world, and above all help the people of Syria, to move forward.”