The former EU trade commissioner Peter Mandelson has accused eurosceptic politicians of "operating a sort of Sopranos-style protection racket inside the Conservative party".
He told BBC1's Andrew Marr Show:
The UK isolation party and their fellow travellers in the Conservative party are ... operating a sort of Sopranos-style protection racket inside the Conservative party. They are saying ... 'give us what we're demanding or we're going to burn your home down'. In my view the Prime Minister has got to say 'enough is enough'.
David Cameron sent an email to members of his Conservative Party urging unity.
The Tory grandee Lord Howe has accused the Prime Minister of 'losing control' of his party as divisions over Europe come to the fore.
The Conservative Party has moved to head off a row with grassroots activists after a Tory ally reportedly described them as "loons".