Ministers should be allowed to campaign for the UK to exit the European Union in an in-out referendum, Boris Johnson has said.
The Mayor of London said it would be "safer and more harmonious" for David Cameron to allow ministers to campaign on the opposing side.
Mr Cameron was criticised yesterday for claiming that he had been "misinterpreted" when he appeared to suggest ministers would be forced to quit if they called for a No vote.
Asked if ministers should be allowed to campaign for "Brexit" and keep their posts, Mr Johnson told LBC's Nick Ferrari: "I don't see why not myself."
He added: "Do you really need to bind everybody in?"
It would be "wrong" to give 16 and 17-year-old's the vote in the EU referendum, Foreign Secretary Philip Hammond has said.
Speaking in the Commons during a debate on the European Union Referendum Bill, Mr Hammond said: "Some will argue that we should extend the franchise further to 16 and 17-year-olds perhaps or even to citizens of other EU countries resident here. We do not agree."
Shadow foreign secretary Hilary Benn confirmed Labour would seek an amendment to the Bill to allow 16 and 17-year-old's to vote.
"It is the same old excuse of an argument against giving people a say and it's completely at odds with the other rights we already give to 16 and 17-year-olds, which include the right to work, pay tax, join the armed forces," Mr Benn said.
Mr Hammond also confirmed that the referendum could be held earlier than planned if renegotiation is completed ahead of schedule.
Prime Minister David Cameron has insisted he was "misinterpreted" over comments which appeared to suggest government minister could be sacked if they want to campaign for Britain to leave the EU.
During a speech at the G7 summit in Germany, he said:
If you want to be part of the government you have to take the view that we are engaged in an exercise of renegotiation to have a referendum and that will lead to a successful outcome.
Everyone in government has signed up to the programme set out in the Conservative manifesto.
A Downing Street spokeswoman said he had been referring only to the need for ministers to observe the rule of collective responsibility during the renegotiation, and had not been speaking about the referendum campaign itself.
She said he would not "get into any hypotheticals" regarding his approach to the referendum, which has been promised by the end of 2017.
After confusion over his statement on how ministers will be expected to behave over the EU referendum and possible renegotiation David Cameron has said his party has 'real unity' over the issues at stake.
Answering a question from ITV News Deputy Political Editor Chris Ship the Prime Minister said "I think the Conservative party is delighted that we've got a renegotiation reform and referendum agenda, there's complete unity about that."
There's real unity behind the renegotiate and referendum strategy, which is right for the country, which is right for the country.
David Davis has said he welcomes the news that the Prime Minister has clarified his comments in which he appeared to say ministers would have to vote with him in the EU referendum.
It' a welcome reinterpretation of his words.
You cannot reasonably stop ministers from voting the way they want to, campaigning the way they want to and speaking the way they want to.
Otherwise that would not be a fair referendum.
Downing Street has attempted to distance itself from comments that implied David Cameron had given Ministers an ultimatum over the Eu referendum.
ITV News Deputy Political Editor Chris Ship reports on what seems like a u-turn from the government.
No10 in full fudge mode. Now say #EURef collective responsibility only applies to renegotiation period-no decision on what ministers do then
So this is either 'mass interpretation' of Cameron's words yesterday. Or No10 saw the fallout-and screeched the vehicle in a big fat U shape
David Davis says the Prime Minister's move to force Ministers to quit if they wanted to campaign No in the EU referendum was "rather unwise" and predicted it would trigger resignations from the Government.
Conservative Mr Davis told BBC Radio 4's Today programme:
This is a once-in-a-lifetime, history-changing event. For many people, it's the reason they got into politics. Not mine, but for some it is.
And yet the only people who will not have the freedom to vote and speak on it, according to this, are ministers in the Government, which, of itself, is extraordinary.
That will likely lead, I'm sorry to say, to some people resigning from the Government or being fired
Ministers will be forced to quit if they want to campaign for Britain to pull out of the European Union, David Cameron has warned.
The Prime Minister said the Government "would not be neutral" on the issue of whether the UK quits Europe.
And he insisted that everyone in his administration is signed up to his strategy to allow them to recommend a Yes vote.
Speaking at the G7 summit in Germany, Mr Cameron said: "If you want to be part of the Government you have to take the view that we are engaged in an exercise of renegotiation to have a referendum and that will lead to a successful outcome.
"Everyone in government has signed up to the programme set out in the Conservative manifesto."
His comments come after more than 50 Tory MPs said they were set to lead the EU exit if Cameron fails to secure radical reforms of the UK's ties to Brussels.
Labour leadership candidate Andy Burnham has warned David Cameron will struggle to get the best deal for Britain "if his party tears itself apart over Europe".
"I will establish a separate 'Labour Yes' campaign, alongside the wider 'in' movement, to learn the lessons of Scotland's independence referendum, " Burnham promised.
"But, unlike David Cameron, I will be strong in defending Britain's place in Europe as the best bet for British business and jobs," he added.