Former German Chancellor Gerhard Schröder has strongly criticised his successor’s handling of the migrant crisis, accusing Angela Merkel of having “no plan” in dealing with the influx.

In an interview with Handelsblatt, the former Social Democrat chancellor accused Ms Merkel of unwittingly encouraging the huge flow of migrants by saying her country would accept all refugees.

“One has the impression as if national borders are no longer relevant. This is dangerous and not correct,” he said.

Germany recorded over one million asylum applications in 2015, with some saying the figures may under-estimate the true scale as many potential applicants would not yet have registered.

Last week, Development Minister Gerd Müller said up to 10 million migrants may eventually come to Europe as many had not even begun to make the journey from refugee camps in the Middle East. There will also be a huge surge in the numbers coming from North Africa, he added.

“The biggest movements are ahead: Africa’s population will double in the coming decades,” he said. “In the Sahara up to one million people have died trying to escape.”

Mr Schröder said that Angela Merkel made a big mistake not being clear that any welcome for migrants would only be temporary.

“Mrs Merkel should have emphasised that this is a one-off relief, and otherwise the existing rules still apply,” he said.

He maintained, however, that she had not planned the crisis, but rather brought it about through sheer naivety: “My impression is that the chancellor had a lot of heart, but no plan.”

The intervention of a left-wing Social Democrat will only add pressure on Mrs Merkel’s increasingly unpopular immigration policies.

Last month, a former communist professor warned that “everything dear to [Europeans]” would disappear thanks to migration.

Jörg Baberowski, a respected historian, said: “…I do believe that something fundamentally new is happening that will change our country very much. Germany will no longer stay out of wars and conflicts. And the Germany that we know it will disappear by the mass immigration.”

Gerhard Schröder served as German chancellor for seven years from 1998 to 2005 when he was defeated by Angela Merkel.