A woman invited to parliament to attend the opening of an exhibition was asked to scratch off an old “stop Brexit” sticker from the top of her laptop before entering, the Guardian has been told. Anna Betz, 66, a retired social worker, was told she had to remove the sticker in case she held the computer in the air to stage a protest. Betz, who was eventually allowed on to the parliamentary estate after covering the sticker on her laptop with a piece of masking tape, said she was baffled by the security procedures. Parliamentary authorities defended the protocols, saying “political or offensive slogan materials” were prohibited inside. The incident happened on Monday evening at the main public entrance to parliament as Betz arrived for the opening of an exhibition about a research project on which she had worked connected to tackling memory loss, hosted by the Liberal Democrat deputy leader, Daisy Cooper. All non-passholders have to go through airport-style security, involving bag scanners and metal-detecting arches, before being allowed inside. Betz, who lives in Brighton, was carrying a small bag for an overnight stay in London, with items including her laptop. A security official took out the device, spotted the sticker and said it had to be removed. “I asked him what he meant,” said Betz, who is related to a Guardian staff member but whose attendance at parliament was connected to her former work with the Camden Memory Service health project. “I didn’t understand what the problem was. He said: ‘You could take your laptop out and demonstrate against Brexit.’ I told him: ‘I’m here for the opening of an exhibition.’” The guard insisted she had to remove the sticker. “I asked how – it had been there for years. He said: ‘You have to use your fingernails.’ I tried but it was so old I couldn’t. I told him I’d tried my best and it was’t working, so what was I meant to do?” Eventually another guard was consulted and she was given a small strip of masking tape to place on the laptop lid. Betz said: “You could pull the masking tape off really easily, but I put it over the sticker. I said: ‘Are you satisfied now?’ He just looked at me and let me go through. It all made no sense. This was a small sticker on my laptop, inside my bag. I just thought: how mad. I’m not sure he understood why he was doing it.” A parliament spokesperson said: “We recognise the importance of democratic access to the Houses of Parliament and facilitate the visits of thousands of people to the estate each week. We have robust security processes in place that help prevent incidents and keep everyone on the estate safe.” The spokesperson said a list of restricted items could be viewed on the parliament website. As well as items such as knives and tools, it includes banners, flags and “political or offensive slogan materials including clothing”.