Brussels last night warned that whoever replaces Theresa May would not be able to make changes to her Brexit deal.
And EU leaders, officials and diplomats all agreed that the chances of a No Deal Brexit had now dramatically increased.
The stage is now set for a showdown if Boris Johnson or another hardline Brexiteer, such as Dominic Raab, wins the keys to Number 10.
Her voice cracking, Mrs May said it had been the 'honour of my life' to be PM, and she hoped she would not be the last woman to lead the country
In the running: Boris Johnson split from his wife Marina and is in a relationship with former Conservative staffer Carrie Symonds
In the running: Dominic Raab has become a cheerleader for a hard Brexit since stepping down as Brexit secretary in November
Guy Verhofstadt, who heads negotiations for the European Parliament, threw down the gauntlet to Mr Johnson and told the Daily Mail: ‘He is famous across Europe for making promises during the Brexit referendum that were undeliverable.
‘The current withdrawal agreement remains the only deal on the table. Why it remains blocked is a matter for the British people and their elected representatives.’
A spokesman for EU Commission president Jean-Claude Juncker said ‘there is no change’ to Brussels’s position and Dutch prime minister Mark Rutte said ‘the withdrawal agreement is not up for renegotiation’ adding that Mrs May’s resignation had ‘increased rather than decreased’ uncertainty.
Irish prime minister Leo Varadkar said: ‘This is a period of danger for us, but whatever happens we’re going to make sure that we hold our nerve.
‘In the next few months we could see the election of a hardline Eurosceptic prime minister who wants to repudiate the withdrawal agreement and go for No Deal or we may even see a new British government that wants to see a closer relationship with the EU and possibly even a second referendum.’
His deputy, Simon Coveney, added: ‘Regardless of who the British Prime Minister is, the EU position will remain that the withdrawal agreement took two-and-a-half years to negotiate, and it’s closed.’
But Spanish government spokesman Isabel Celaa warned that ‘a hard Brexit appears to be a reality that is near impossible to stop’.
She added that the UK Government and MPs would be ‘solely responsible for a No Deal exit and its consequences’.
EU diplomats said they most feared a hardline Brexiteer taking over, but predicted that if one did they would be broken down by Brussels.
One said: ‘I think the new PM, if everyone tells them what No Deal means, will be concerned with the ramifications.
The Mays left Downing Street via the back door after she made her dramatic departure announcement today
