The Deputy Prime Minister Nick Clegg has said he would support David Cameron if he decided to veto a deal on the EU budget.
Mr Cameron is planning to push for a freeze to the budget in real terms, but some believe this will be a hard sell when many EU states support a rise in spending.
More top news
Right-wing think tank warns leaving the UK in order to remain in the EU would turn Scotland into 'Greece without the sun'.
A poll of Britain's largest trade union, seen by ITV News, indicates more than half of those surveyed think the Labour leader should quit.
Party rules state the new Tory leader will be announced on 9 September, but the way things are going it could all happen much sooner.