Boris Johnson has questioned key parts of the Prime Minister's proposed deal with the European Union, asking why David Cameron did not "try harder" to regain control of the UK's borders.
The London mayor, who is being courted by campaigners on both sides of the argument ahead of the in/out referendum on EU membership, insisted he would like to remain in a reformed bloc but would "wait until you see the whites of their eyes" before making his decision.
Mr Cameron faces days of intense negotiation in the run-up to the February 18-19 Brussels summit where he hopes to reach agreement on the deal proposed by European Council president Donald Tusk.
Mr Johnson used his Daily Telegraph column to warn that the proposals were not agreed among EU leaders and the European Parliament's president Martin Schulz "has already said that he wants to unscramble them".
"This is the moment to stiffen the sinews, summon up the blood, squint down the barrel and only when you see the whites of their eyes should you finally let fly and decide whether to stay or leave the EU; because the arguments are as finely balanced as they have ever been," he said.
On the issue of protection for the UK and other countries outside the eurozone, Mr Osborne asked: "Is it a concession by them, or by us? The salient point appears to be that the UK will not be able to block moves to create a fiscal union - a deeply anti-democratic exercise."
On the competitiveness agenda "the language is excellent" but "Why are we not insisting on a timetable for a real single market in services?"
Mr Osborne questioned Mr Cameron about measures to protect sovereignty in the Commons last week, and he wrote that in the proposed deal "it looks as though the Prime Minister has done better than many expected".
But he questioned how "bankable" the promises were and whether an "intimidating" measure could be constructed to counter European judges and bureaucrats.
"Are we talking bazooka or popgun?"
He criticised a lack of ambition in measures aimed at curbing migration, where the Prime Minister has secured the offer of an "emergency brake" to restrict access to in-work benefits for EU migrants.
"Why didn't we try harder to recapture control of our borders, rather than stick at this minor (if worthwhile) change to the law on benefits?"
Mr Johnson said that he would give his views "if and when a deal is done".
Among the arguments for remaining in the EU were the value of the single market, and the uncertainty caused by leaving the 28-member bloc.
He added that "history shows that they need us" and leaving would send a "very negative signal" for Europe, particularly the East.
But against those arguments were the "woeful defects" of the "wasteful, expensive and occasional corrupt" EU.
"So there is the dilemma in a nutshell: Britain in the EU good, in so far as that means helping to shape the destiny of a troubled continent in uncertain times, while trading freely with our partners," he said.
"Britain in the EU bad, in so far as it is a political project whose destiny of ever closer union we don't accept and whose lust to regulate we can't stop.
"That is why for the last couple of years I have argued that we would be - on the whole - better off in a reformed EU, but that Britain could have a great future outside. In deciding how to vote I (and I expect a few others) will want to know whether we have genuinely achieved any reform, and whether there is the prospect of any more."
His intervention came after Mr Cameron was warned his proposed deal on benefits could lead to a surge in the number coming to the UK before the "emergency brake" mechanism is applied.
Former leadership contender David Davis warned that coverage of the proposed curbs on benefits on the continent would act as an incentive to workers to head to the UK over the coming months before a deal can be implemented.
Mr Davis said since the proposed deal was set out "Eastern European newspapers have carried numerous stories about in-work benefits and the plans to terminate them for the first four years after a migrants arrival in the UK" while Brussels has suggested that the scheme could take 12 months to implement.
"Under such circumstances the incentive for anybody wishing to come to live in the UK will be to come as quickly as possible to beat the deadline when any such restrictions come into effect," former Europe minister Mr Davis said.
"Accordingly we are likely to see a surge in migrants in the next 12 months."