Legislation paving the way for a referendum on Britain's membership of the European Union will be debated by MPs as David Cameron seeks to maintain Conservative Party unity over efforts to negotiate better terms with Brussels.

The Prime Minister is expected to be in the Commons for the second reading of the European Union Referendum Bill, a day after facing backbench anger over suggestions ministers who campaign to leave the EU could be sacked.

Attending a G7 summit in Germany, Mr Cameron sought to blame the media for "misinterpreting" his position - insisting he will not announce until nearer the time of the public vote whether senior colleagues will be bound by collective responsibility.

The Bill, which puts into law the Conservative manifesto promise to put an in/out choice to the electorate by the end of 2017, will have no trouble getting over its first parliamentary hurdle after Labour dropped its pre-general election opposition to the policy.

But Mr Cameron will face pressure from eurosceptic Tories and opposition parties to make changes at later stages - most significantly in the Lords where the Tories do not have a majority - despite appealing to his own backbenchers not to seek amendments.

The PM is walking a political tightrope as he seeks to win over support in Europe for his bid to reform the UK's links to Brussels while keeping his party united on the deeply divisive issue.

At least 50 Tory MPs, including former cabinet ministers Owen Paterson and John Redwood, have signed up to the newly formed Conservatives for Britain (CfB) group, which says it is ready to campaign for a British exit even if the PM believes that staying in would be in the national interest.

They are particularly critical of a move to allow unlimited spending by public bodies on promoting a Yes vote - including the Government and the European Commission - throughout the campaign.

They have been bolstered by the Electoral Commission watchdog which said it was "disappointed and concerned" by the proposal to lift campaign restrictions, warning that it undermined regulation and "could give an unfair advantage to one side of the argument".

Other potential parliamentary flashpoints surround the chosen question - "Should the United Kingdom remain a member of the European Union?" - and the timing of the vote.

Eurosceptics say the wording has been chosen deliberately to maximise the chance of a Yes vote - with the Commission previously suggesting it could be more balanced to give a more explicit choice between remaining in the EU or leaving.

Mr Cameron downplayed the Commission's concerns that holding it on the same day as other elections - such as the devolved and local polls in May 2017 - would not give the public "space to engage fully with the referendum issues".

"Personally, I think the British public are quite capable of going to a polling booth and making two important decisions rather than just one, and I think the evidence has shown that," he said, insisting the date would be determined by the progress of renegotiations.

He plans to continue that push in a series of one-on-one talks with EU counterparts on the fringes of a Europe/Latin America meeting in Brussels tomorrow, ahead of a European Council summit at the end of the month.

Labour, along with the Scottish National Party, is to press for the vote to be extended to 16 and 17-year-olds, following the example of last year's Scottish independence referendum.

The SNP, now the third-biggest party in the Commons, also wants a "lock" written into the legislation so the UK would only leave the EU if there was a majority in favour of doing so in each of the four nations of the UK.

Mr Cameron indicated that he does not want to see Conservative MPs putting forward amendments, saying: "We'll be putting forward the Bill that we think is the right Bill. It will be a matter for Parliament to discuss and debate it."

He denied that the Tories' difficulties over Europe were an echo of the party's 1990s splits, insisting there was "real unity" behind his plan.

In the UK's last referendum on Europe in 1975, then-prime minister Harold Wilson avoided splitting his party on the emotive issue by allowing ministers to campaign on opposing sides.

Senior Conservatives cautioned the PM against trying to force ministers to toe the line on an issue which is believed to have split the Cabinet.

In a briefing sent to MPs, the Electoral Commission strongly criticised the move to lift campaign spending restrictions on public bodies in the final 28 days before the country decides.

"In the Commission's view, there is a risk that the use of significant amounts of public money for promotional activity could give an unfair advantage to one side of the argument," it said.

"It would also undermine the principle of having spending limits for registered campaigners if governments were able to spend unlimited funds on paid advertising during the period when campaigners were restricted in the amount they could spend.

"This has the potential to be particularly significant in the case of a referendum on the UK's membership of the EU where there would be four governments with views on the issue being debated, as well as local authorities who may have strong interests in promoting a particular outcome."

Former cabinet minister Andrew Mitchell warned that preventing Tories campaigning to leave would undermine any vote to stay in.

He told BBC2's Newsnight: "It would be a big mistake to try and whip in people who want to leave the European Union in the Conservative Party and the reason is this: if you do that, at the end of the day when we need people to respect the democratic decision of the British people in the referendum, there will be Tories who will feel that it was not free and fair because there were artificial restraints put on those who want to leave.

"Let people follow their convictions on an issue which is very fissile and difficult for the Conservative Party. Let people campaign, let people argue for the things they strongly believe in, let the British public decide and then it is a democratic decision made by the British people and hopefully all Conservatives will accept it."

Asked if Mr Cameron could secure sufficient reforms to satisfy hardline eurosceptics, he said: "We'll have to see, we don't know.

"But what we do know is that there's a very sensible road map here that means the Tory party does not need to go off-piste and it is to have a renegotiation and then everyone can decide for themselves, in an in-out referendum, whether we stay or we go.

"There is no one better than than the Prime Minister at negotiating with his European colleagues. He has already shown that he knows how to do it. He manages to extract a lot of goodwill."