Tory and crossbench peers sign Labour-Lib Dem motion to allow 16- and 17-year-olds to take part before they vote on issue next week

The likelihood of the House of Lords supporting a lowering of the minimum voting age in the European Union referendum has increased after Conservative and crossbench peers signed a Labour and Liberal Democrat-backed amendment allowing 16- and 17-year-olds to vote.

The Lords are due to vote on the issue next Wednesday and it would be a major headache for the government if peers gave it their backing because it might take the Electoral Commission up to nine months to prepare for an enlarged electorate.

That would effectively ensure that the referendum could not be held until autumn 2016 at the earliest, reducing David Cameron’s freedom of manoeuvre on the date. Downing Street would like to retain the option of a summer referendum if the prime minister’s negotiations with Britain’s EU partners go quickly and successfully.

The Lords motion has been signed by the Conservative peer Lord Tugendhat and by the crossbencher Lord Hannay, a former senior diplomat and UK representative to the EU.

If it does win Lords support, Cameron will face the dilemma of whether to try to use his narrow Commons majority to force peers to rethink. With Cameron facing many European battles with his Eurosceptic backbenchers, he may decide this is one European battle it is not worth fighting

Many pro-Europeans are leaning towards giving 16- and 17-year-olds the vote partly because there is some polling evidence that they will support remaining in the EU. A precedent has been set in Scotland where the voting age was lowered in the referendum on Scottish independence. Young people appeared to be highly engaged by the issue, even if turnout was slightly lower than among older age groups.

The Scottish parliament has voted to extend the franchise permanently in Scotland, putting pressure on England and Wales to follow suit.

In a briefing to peers, the Electoral Commission wrote: “Any changes to the franchise for the referendum on the UK’s membership of the European Union should be clear in sufficient time to enable all those who are newly eligible to vote to take the steps they need to successfully register and participate in the referendum.”