The UK's Pensions Regulator is pursuing the insolvent Canadian group, Nortel Networks, over a debt in its UK pension scheme that it has put at £2.1bn (€2.4bn) - one of the largest sums it has ever sought, and a fresh indication of its willingness to flex its muscle internationally.

The UK agency served a "warning notice" against the Canadian company in January, according to court papers published last week by accountancy firm Ernst & Young, which is acting as 'court monitor', or overseer of Nortel while it is in bankruptcy protection.