The Liberal Democrats have issued their first formal coalition demand, with Nick Clegg insisting on his education funding plans being implemented in full in any future deal.
In a break from his previous refusal to use the term "red line", Mr Clegg said the commitment to increase the education budget for two-to-19 year-olds to more than £55 billion by 2020 will be a non-negotiable issue in any agreement with the Tories or Labour.
The policy would mean spending £5 billion a year more than the Tories or £2.5 billion a year more than Labour by the end of the parliament.
The Lib Dem leader will set out further deal breakers in coming days as he positions his party as a potential partner for either David Cameron or Ed Miliband.
Acknowledging that there were "Premier League" policies which would be red lines and other policies which could be dropped in any coalition talks, Mr Clegg said it was important to give voters certainty about the Lib Dems' demands.
Speaking to reporters on his battle bus he said: "As the outcome of the election appears increasingly uncertain and the risk of instability immediately afterwards appears increasingly worrisome, I have decided I think we need to be increasingly clear and certain and categorical with people about what the Liberal Democrats will do after May 7 come hell or high water if people vote for the Liberal Democrats.
"In other words, I think the more fluid and uncertain the possible outcome is becoming, I think the more we need to provide people with good reason, with fixed points as far as what Liberal Democrats would do after the election.
"I have been quite reluctant to talk about the language of 'deal breakers' and 'red lines' and so on.
"But I think it is now right in the final stages of the election to set out some core deal breakers for us."
Mr Clegg, who has been dogged by the party's U-turn over tuition fees, accepted the need for clarity about his position on key policies.
"I think when there are so many different permutations, so much speculation about who might talk to whom after May 7, I think it is only fair that we say to the British people 'if you vote for the Liberal Democrats this is what you will get, no ifs, no buts, this is a deal breaker'."
Mr Clegg has repeatedly stressed that the five policy areas listed on the front page of his party's manifesto would be the ones he will "fight tooth and nail" for in coalition negotiations and are likely to feature in his future announcements on red lines.
As well as the commitment on education funding, the manifesto's cover includes aspirations to balance the books fairly, protect the environment, raise the income tax personal allowance to £12,500 and invest £8 billion to improve the NHS.
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