DOWNING Street has brushed aside David Mundell’s insistence that the referendum on Britain’s membership of the European Union could be held in June.

The Scottish Secretary has become the most senior Tory minister to back a June poll, which would follow on soon after the Scottish parliamentary elections in May.

“There isn't an argument against having the referendum in June,” declared Mr Mundell, who added: “There is a strong argument to have it then, so we can get on, have the debate and get the referendum out of the way.”

But when asked about the Secretary of State’s remarks, David Cameron’s spokeswoman said what mattered was getting the right reforms, which “would be driven by substance not by schedule”.

Earlier this week, Philip Hammond, the Foreign Secretary, spoke out against having the poll in July or August and said it would be "odd" to hold a vote in the first half of September "when people have just come back from holidays".

If the Cabinet ministers are right in their thinking, then the window of opportunity for David Cameron, who has indicated he would like a summer poll, is very limited indeed.

A six-week campaign starting after the May 5 elections in Scotland, England, Wales and London means the earliest possible date is Thursday June 16. But on this day England, Wales and Northern Ireland take part in the football European championship.

The last week in June sees the start of the Scottish school summer holidays. The English school summer holidays begin in July and August is the month when many Britons go abroad for their summer break.

Thus, if the Prime Minister wishes to avoid a September referendum – which would have to take place early in the month to avoid clashing with the party conference season – then the optimum date for the Euro poll is Thursday June 23.

Mr Cameron’s spokeswoman made clear she did not have a crystal ball and, when asked if June 23 was indeed his preferred date, replied: “His preference is to secure the right outcome in this renegotiation to address the concerns of the British people.”

Elsewhere, Donald Tusk, the European Council President, boosted the PM's hopes of a deal at the Brussels summit in February, saying it was "not easy but possible" that agreement could be reached between the other 27 member states next month.

He made clear a "concrete proposal" for EU reform would be tabled within weeks.

It came as Mr Hammond indicated that alternatives to a proposed four-year benefits ban to stem the flow of EU migrant workers to Britain, the principal stumbling block to a deal, were now being considered.

Reporting to the European Parliament about the last EU summit in December, Mr Tusk insisted there could be "no compromise on fundamental values like non-discrimination and free movement".

But he told MEPs: "At the same time, I will do everything in my power to find a satisfactory solution, also for the British side.

"As of today, the result of the referendum is more unpredictable than ever before. Time is of essence here. And this is why I will work hard to strike a deal in February. It will not be easy but it is still possible," he added.