The defence ministry has submitted a request for 4.7 billion euros ($5.0 billion) in extra spending to buy the U212CD submarines from Germany's Thyssenkrupp Marine Systems, according to the source.

"We are treating the proposal to purchase additional submarines with particular attention because of its importance for security policy but also because of the high level of funds required," said the MP.

Germany had already ordered two of the same submarines from Thyssenkrupp Marine Systems as part of a joint project with Norway, which has also ordered submarines.

Berlin has been looking to revamp its depleted armed forces in the wake of Russia's invasion of Ukraine and has pledged huge sums to boost the military to combat growing threats.

However, a special spending request for the submarines was necessary because they are not included in Germany's budget planning for 2024 and beyond.

In its request, the ministry has argued that Germany "would not be able to meet the new NATO requirements for better protection of the alliance's northern flank without the additional four submarines", according to Der Spiegel magazine.

Chancellor Olaf Scholz is leading a minority government until elections expected on February 23.

To secure the funding now, Pistorius would need the support of the conservative opposition.