LABOUR believes it can win the battle to enable Britain's 16 and 17-year-olds to vote in the EU referendum as the SNP hit out at the possibility that the poll could take place on the same day as the May 2016 Holyrood election.

During robust exchanges in the Commons, Harriet Harman, Labour's acting leader, challenged David Cameron over extending the franchise to allow more than 1m young people in the UK to take part in the key EU vote.

"Why will the Prime Minister not let 16 and 17-year-olds vote? This is about the future of our country. They did in the Scottish referendum. It is their future too," declared the London MP.

Mr Cameron replied by saying MPs "should vote on that issue" but pointed out the Conservative manifesto was clear and his position was clear, namely "we should stick with the current franchise at 18".

Senior Labour sources made clear the Opposition would ensure that there would indeed be a vote by tabling an amendment to the EU Referendum Bill going through Westminster; the detailed committee stage begins next Tuesday.

One said: "We are determined to win this and we think we can. There will definitely be an amendment. It's illogical that 16 and 17-year-olds can vote on the future of Scotland in the UK but not on the future of the UK in Europe."

He noted that the Government had only a majority of 12 and that it was known some Conservatives back giving young people a vote in the referendum. "It could be close," he added.

Meantime, the PM sparked controversy when he conspicuously failed to rule out holding the EU referendum on the same day as another poll, namely, the Holyrood election next May.

While the Government has said it wants to hold the in/out referendum by December 2017, many believe Mr Cameron would like to stage it as early as possible and preferably in 2016.

Ms Harman pointed out how the Electoral Commission, the elections watchdog, had made clear the EU referendum should not be held on the same day as any other election and told Mr Cameron: "We strongly agree. This is an important constitutional issue that should be considered on its own. Will the Prime Minister guarantee a separate voting day for the referendum?"

But Mr Cameron replied: "The timing of the referendum should be determined by the timing of the renegotiation; when the renegotiation is complete, we set a date for the referendum. I do not think it should be determined by the timing of other elections.

"For instance, it was possible to have the AV referendum and other elections on the same day. People are capable of making two decisions," he added.

But after PMQs, the SNP's Stephen Gethins expressed alarm. "Holding the EU referendum on the same day as the Scottish elections would be simply unacceptable and it is extremely disappointing that David Cameron has failed to rule this out.

"The Electoral Commission has already stated that referendum polling day should not be shared with any other elections and the UK Government previously signed up to this principle in the Edinburgh Agreement ahead of the independence referendum. Failure to follow this advice now will be a failure to ensure best practice," added the MP for North East Fife.

If 16 and 17-year-olds did not get to vote on the EU referendum and it was held on the same day as the Holyrood elections, then this could cause administrative problems as young people in Scotland and resident EU citizens would be able to vote on the latter north of the border but not on the former.

Mr Cameron defended his decision to lift the purdah restrictions on Government activity during the EU referendum campaign, saying the decision not to do so in the run-up to last year's Scottish independence poll was "a ridiculous situation, which is why we have proposed changes to the purdah rules".

However, some MPs fear the Government could greatly influence the argument in favour of the Yes campaign. Ms Harman said there must be a legal framework in the Bill and that "ministerial restraint" could not be relied upon.