The pub is near Edinburgh Castle and other tourist attractions

Audrey Ollason, who runs Edinburgh's Ensign Ewart, said she was forced into the move because of the dozens of tourists who queue up each day.

She claimed the problem got so bad her paying customers were having to wait for the loo.

Now Ms Ollason only issues those who buy a drink with the secret combination to open the door locks.

She said the pub, which is situated on the Royal Mile and is the closest pub to Edinburgh castle, was always packed with passers-by who nipped in for the ladies or gents without buying a drink.

Ms Ollason said: "During the festival we would have dozens of people traipsing in and out to the detriment of our own customers not being able to use the toilet.

We pay for the electricity and water so why should we pay for non-customers to use our facilities Audrey Ollason Pub landlady

"It has been a huge problem for us over the last 18 years and we thought about it long and hard.

"We installed the system because we simply couldn't cope with another Tattoo.

"We are treated like a public convenience even though there are toilets up at Castlehill. The problem is they are not well signposted.

"We pay for the electricity and water so why should we pay for non-customers to use our facilities?"

But politicians have slammed the move by the pub, which is named after Ensign Charles Ewart who captured the French standard at the Battle of Waterloo, and claim it gives tourists an image of Edinburgh as unwelcoming and unfriendly.

Tom Ponton, Liberal Democrat councillor and owner of the Oz Bar in Edinburgh, said the move was unnecessary.

'Occupational hazard'

He said: "It's absolutely daft and unnecessary. I don't know how much a system like this would cost but I think it would buy an awful lot of toilet paper.

"It is an occupational hazard of having a pub in the city centre that people will want to come in and use the facilities without buying a drink. It is difficult to refuse them.

"However, I can sympathise with the publican and I really feel the council should be allocating more money to the provision of public toilets in the city centre."

The pub's landlady says dozens of tourists to Edinburgh Castle use her toilets

The SNP's Kenny MacAskill added: "If someone walked in as a stranger or with a small child needing to go to the toilet, I would expect them to extend the courtesy of letting them use them.

"It is a matter of balance though. If we need to look at better provision of public amenities and signage to them then we should do so.

"It is a business and it is not fair on the pub or their regular clientele if there are hordes of people walking in off the street to use the toilets."

A spokeswoman for Edinburgh City Council said there are public toilets nearby but admitted they are not open 24 hours.

She said: "The toilets are signposted and located centrally for tourists to find easily. They are not open 24 hours a day for staff and safety reasons.