'John McDonnell last night blasted Theresa May's Brexit strategy - comparing it to Neville Chamberlain's appeasement of Adolf Hitler.
The shadow chancellor said the PM's plan is the 'worst of all worlds' which would fail to provide stability for businesses or workers.
And he confirmed Labour would be voting against the deal if a customs union with the EU is not permanent - despite the prospect of a revolt by backbenchers who have said they could back the PM.
Mr McDonnell said Mrs May will come back hailing a deal, but it will be 'more like Neville Chamberlain' than Winston Churchill and will 'disintegrate' over time.
His jibe is a reference to when Mr Chamberlain signed the Munich Agreement with Hitler in 1938 and then proudly toured the country declaring he had secured 'peace for our time'.
Within a year Britain and the Allies were at war with Germany after Hitler had marched through much of Europe conquering vast swathes of the continent.
Appearing on BBC's Newsnight last night, Mr McDonnell said he 'can't see' Labour backing Mrs May's deal as 'I think it would be the worst of all worlds'.
He said: 'What I am worried about is that Theresa May comes back waving a piece of paper, and it won't be like Winston Churchill, it will be more like Neville Chamberlain, and we will see peace for our time that will then disintegrate over time.
'I'd rather she just came back and told us - if she can get a deal that protects jobs and the economy we will vote for it.'
Read more: John McDonnell says Labour will vote AGAINST May's Brexit plan if a customs union with the EU is not permanent as he warns her deal will be like the appeasement of Hitler
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