An underground passage between servants’ quarters and the vast mansion it belongs to has been extended illegally without planning permission.

The mysterious owners of Witanhurst, off Highgate West Hill in Highgate Village, have applied for planning permission retrospectively after carrying out works on a subterranean link between an enormous double-storey basement and the main house, which is classed among the largest in London.

The passage links the “service areas” in the main house with an underground swimming pool in the basement extension.

Safran Holdings Ltd has now lodged plans with Camden Council for retrospective permission to extend the underground link.

Susan Rose, chairwoman of Highgate Conservation Area Advisory Committee, said: “I’m not in the least surprised.

“It’s yet more disruption, more excavation.”

She added: “It just shows the lack of respect for the planning system.”

The owners began building their vast £50million underground palace, complete with massage room, basement car park, staff quarters, sauna, jacuzzi and 70ft swimming pool in 2010.