The BBC spent nearly £100million of public money collecting the TV licence fee last year. But it still managed to lose more than £250million in revenues from people who dodged the £145.50 charge.

The corporation spent almost £16million sending threatening letters and emails, and stepped up the number of visits by TV licence enforcement officers.

It also deployed hi-tech detection vans, designed to catch un-licensed people watching programmes on the BBC iPlayer.

But despite these extra efforts, the number of people dodging the so-called TV tax rose.

According to a damning report by the National Audit Office, up to seven percent of people who were supposed to buy a licence did not pay up – equivalent to about one in 14.

That was an increase from the previous year, when between five and six percent of TV viewers refused to part with their cash.

The corporation stepped up the number of visits by TV licence enforcement officers

Every household or office in the country requires a licence to watch TV, regardless of whether viewers are watching programmes live on a traditional television or catching up on older BBC shows on the iPlayer.

Many households did not pay the charge because their budgets were severely under pressure, the NAO report said.

One might imagine that the surge in people watching TV without a licence would make it easier for the BBC to find TV licence evaders.

But the number of people actually caught dodging the charge fell by nearly a fifth since 2010, the NAO said.

Capita, one of the two companies the BBC employs to do most of its leg work, added that it is getting harder to catch licence fee dodgers than in the past, because inspectors are afraid of being beaten up.

The firm - which has been accused of using bullying tactics in the past - said that more of its inspectors were suffering physical and verbal abuse, and were sometimes beaten up so badly they had to be hospitalised.

Every household or office in the country requires a licence to watch TV, regardless of whether viewers are watching programmes live or catching up on iPlayer

Many people are also filming or photographing inspectors who come to their houses, putting would-be enforcement officers off the job.

The NAO said it was ‘worrying’ that the BBC failed to catch so many licence fee dodgers last year.

And it added that the BBC is now on course to miss its own targets to bring the evasion rate below four percent by 2020.

It also ordered the BBC to work harder to get value for money from the £99.6million it spends on collecting its £3.7billion-a-year of licence fee funds.

At the moment, most of the money it spends on collecting goes on call centres, TV licence inspectors, detection vans, over the counter services, as well as producing and posting nearly one million threatening letters a year.

Amyas Morse, NAO chief executive, said: ‘The BBC has scope to improve further the value for money of licence fee collection’.

He praised the BBC for reducing the amount it spends on licence fee collection from £101million in the year to April 2015.

A BBC spokesman said: ‘Despite most people abiding by the law and more licences being in force than ever before, there has always been a small minority who refuse to pay the licence fee, so we’ll continue to use the full range of enforcement methods and encourage people to buy a licence at every stage as well as considering any further improvements which can be made.’