A politician from Angela Merkel's party has warned David Cameron he risks losing Scotland and much of the UK’s economic firepower if Britain leaves the European Union.

The Prime Minister is due to celebrate his 49th birthday today by holding talks with the German Chancellor on EU renegotiation.

But Michael Gahler, an MEP with Ms Merkel’s CDU party, issued a stark assessment of the potential repercussions of a 'Brexit'.

“Leaving would re-open the Scotland debate – I warn you on that,” he said, adding: “The city on London is not online (with any exit) they might also leave if (the UK votes to) leave."

He added that the 'the Empire” was “over' and that the UK needed Europe to remain in the 'Champions League' of world economies.

His no-holds-barred warning came as a new cross-party campaign for Britain to quit the EU was launched.

Supporters claimed that ‘Vote Leave’ was now the frontrunner to be designated the official "Out" campaign by the elections watchdog.

A rival umbrella group, Leave EU, has already been set up.

The Electoral Commission will pick one one ‘Remain’ and one 'Leave' campaign.

The chosen groups will have significant advantages - including campaign broadcasts, free mail shots and public funding of up to £600,000, as well as a much higher overall spending limit.

But the decision has already caused controversy after the Commission ruled that there could be no 'Yes' or 'No' campaigns, despite making the opposite ruling in last year’s Scottish independence referendum.

Mr Cameron and Mrs Merkel are expected to discuss deals of Britain's renegotiation demands later.

The war in Syria and the migrant criss are also due to be on the agenda.

Meanwhile, a new por-EU group, called 'Scientists for EU' has also launched.

The advisory board includes Dame Anne Glover, from Aberdeen University, the former chief scientific advisor to the European Commission, a post that was controversially axed last year.

Other scientists involved include Lord Martin Rees, the Astronomer Royal, as well as representation from the Conservative, Labour and Liberal Democrat parties.

Dame Anne said: “The UK benefits hugely from the EU science budget and as a full member of the EU, we can also influence and contribute leadership to the research programmes.

"Without influence and access to this budget we will compromise our competitiveness and will cease to be a magnet for global scientific talent.”