Emmanuel Macron will be strongly tempted to veto another delay to Brexit because of the “deteriorating situation” in the UK, a former top French diplomat is warning.

The French president – who has already threatened not to grant an Article 50 extension – will regard the crisis enveloping Boris Johnson as even “more disturbing” than the preceding events, Pierre Sellal said.

“The situation of the UK as a member state becomes every day more awkward and strained,” the former French ambassador to the EU warned.

Mr Sellal did not rule out a French veto – if, as expected, the prime minister is legally forced to request one – saying the UK was failing to present “a credible acceptable alternative” to the deal it had rejected.

Arguing that Mr Macron would require “a sufficient level of trust”, he told BBC Radio 4: “Maybe what is missing today is this trust about the way your country sees its future with the European Union.

“In this regard, I believe that the situation has been deteriorating. It is very difficult to have the necessary trust that could justify a new examination of a new date.”

If France did veto an extension, the UK would crash out of the EU on 31 October, unless parliament suddenly approved the divorce deal – or revoked Article 50 altogether.

Mr Macron has long been identified as the most hardline EU leader, in contrast to Angela Merkel, the German chancellor, who is willing to give the UK more time.

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It was widely assumed the French president would not “do a de Gaulle” – echoing his predecessor’s veto of the UK joining the then-EEC in the 1960s – if the UK held a general election, or Final Say referendum.

But Mr Sellal said, of a likely autumn election: “It is not a question of personalities – it’s a question of substance.”

He added: “The most important question, and the question that Mr Macron in particular demanded last time, will be for what purpose exactly?”

A UK request to extend Article 50 is looming, because a law to prevent a Halloween crash-out will be passed on Monday, in a crushing defeat for Mr Johnson.

On the same day, the prime minister will fail to trigger the October election he craves – he needs two thirds of MPs to vote to overturn the Fixed-term Parliaments Act on Monday, the magic figure of 434.

But the opposition parties, including Labour, are united in opposing an early poll.