David Cameron has stressed the importance of six-day-a-week deliveries to any address in the UK - after the boss of Royal Mail warned the service was in jeopardy.

Moya Greene told MPs the company faced an "existential crisis" and called on regulators to accelerate a review into the Universal Service Obligation. But her comments were dismissed by Liberal Democrat Business Secretary Vince Cable.

He accused the Royal Mail of "scaremongering" over the threat to the service, which is vital to many rural areas but costly to run.

Earlier, Ms Green accused private delivery firms of "cherry picking" the easiest deliveries. She told the Commons Business Select Committee: "Low cost, high density areas are needed to cross subsidise suburbs and rural areas which are much higher cost.

"If you allow cherry picking you undermine the economics." Ministers faced criticism when they sold off Royal Mail, amid accusations the low price lost taxpayers more than a billion pounds.

Mr Cameron replied: "I know how important the universal service obligation is, particularly in constituencies like yours that include so many islands and far flung communities. It's very important it's maintained."