Boris Johnson has called on the Prime Minister to "roar on" in a "full-scale breakout" of EU reform, insisting that David Cameron has the momentum in Europe.
The Mayor of London, who was elected as MP for Uxbridge in the General Election, said that Europe did not expect Cameron to win the election.
This, he argued, handed Britain the opportunity to push for a deal "that is good for the productiveness and competitiveness of the whole EU economy".
Writing in the Telegraph, the London mayor said:
This is a once in a lifetime opportunity. This is the first time in my adult existence that Britain is leading a campaign for change and reform in the EU that is backed up by the prospect of an in-out referendum.
We cannot be half-hearted; we cannot just throw this chance away. Now is the time to rev up our engines, and fan out through the gap. It is clear that David Cameron is already making the running in Europe.
He said that there were "plenty" of other countries who agreed with Cameron on matters such as migrant benefits, the intrusiveness of the EU and maintaining the sovereignty of national parliaments.
Earlier, Johnson told the BBC:
If [David Cameron] doesn't get the reform that he wants in Brussels - and I think it's very, very, very, very unlikely, but if he were to fail in that ambition - then clearly he would be recommending a no to the British public I would imagine.
It cannot be true that we're going into the negotiation signed up automatically to say yes to whatever the outcome is. It can't be that that is the British negotiating position [...] it's got to be open to Britain to decide there is an alternative future and there could well be an alternative future.