Cardiff's brand new HMRC tax office has been completed in the capital's Central Square, but passers by have noticed that it isn't the city's most environmentally friendly building.
The new Government hub will be home to 4,000 civil servants from several UK Government departments and agencies.
It will also host full UK Cabinet meetings if and when needed to.
The £100m building is 12 storeys high and is one of 16 UK Government hubs being created across the UK.
Workers aren't moving in from their current Llanishen office until later this year, but the lights are on all evening, every single night.
This is how the building looked late on Saturday
Bryony Haynes, coordinator for Cardiff Friends of the Earth said new developments across the country should be setting an example on how to use energy more efficiently.
She said: "When security is not an issue, we should not be leaving lights on all night when we wouldn’t even do it in our own homes.
"Given that we are in a climate crisis, we must ensure that we are making a conscious effort as often as possible to reduce our greenhouse emissions - especially when it is as easy and as simple as flicking off a light switch.
"New developments in Cardiff and across Wales should be setting an example in using energy more efficiently."
She added: "It is a waste of energy when the building isn’t ready for use and may potentially contribute to carbon dioxide emissions directly caused by power consumption.
"The more energy we unnecessarily consume adds to the negative impact on our local and national climate."
But HMRC says the lights must remain on - for now.
A spokesman said: “The Cardiff Regional Centre is a building site and good lighting is essential for security purposes.
“When the building is occupied, a permanent, sustainable solution will be in place.”
