Royal Mail is facing a 'death spiral' over its obligation to deliver post six days a week, according to would-be owner Daniel Kretinsky.

The billionaire – nicknamed the Czech Sphinx – described the duty as 'the most strict in Europe'.

Mr Kretinsky's comments are the strongest hint yet that, should he get his hands on the company, he would look to reform the universal service obligation (USO).

By law, Royal Mail must deliver letters to every address in the UK, six days a week, at the same price. It must also deliver parcels on five days.

But it has continually missed delivery targets and has run up huge losses.

In a rare interview, with Bloomberg TV, Mr Kretinsky said: 'The rules for the universal service obligation in the UK are the most strict and the most rigid in the whole of Europe... that has created the situation where Royal Mail has started to lag behind the competition.

Royal Mail is facing a 'death spiral' over its obligation to deliver post six days a week, according to would-be owner Daniel Kretinsky (file picture)

Daniel Kretinsky (pictured) - nicknamed the Czech Sphinx – described the duty as 'the most strict in Europe

By law, Royal Mail must deliver letters to every address in the UK, six days a week, at the same price (file picture)

'The fight for the Royal Mail is delivering the right services for the right price and hence be competitive and preserve market share. If these conditions are not there, then it goes into a death spiral because of the universal obligation.' 

This week, Royal Mail's parent company, International Distribution Services, accepted a £3.6 billion offer from the tycoon after he made a series of commitments, including promises on jobs and the UK-wide six-day service.

Results published last week show Royal Mail is losing around £1 million a day.

Shareholders will vote on the deal in September, but ministers could block it under the National Security and Investment Act.

Chancellor Jeremy Hunt said: 'We will make sure whoever takes over the Royal Mail, if someone does indeed take it over, that those things that people depend on are safe.'