They may be the most unlikely of sparring partners, but the row between Greenpeace and Lego over the toy manufacturer's relationship with Shell stepped up a notch today.

Since 2012, Lego has partnered with the oil and gas giant to produce Shell-branded toys at petrol stations, in a sponsorship agreement that is estimated to be worth $116m.

"Lego, the biggest toy company in the world, has built its brand on its continued promise of leaving a better world for children. And by teaming up with Shell it's letting kids down," Greenpeace has said.

Greenpeace has held protests at Legoland in the UK, where activists defaced various Lego-built landmarks, angered by Shell's campaign of Arctic drilling.

The NGO has now published a new animated campaign video, entitled LEGO: Everything is NOT awesome, which shows an oil spill seeping from a Lego-built oil rig, before enveloping a Lego-built arctic tundra. The strapline is: "Shell is polluting our children's imaginations."

Lego has responded angrily to the claims, with President Jørgen Vig Knudstorp issuing a statement saying: "The Greenpeace campaign focuses on how Shell operates in a specific part of the world. We firmly believe that this matter must be handled between Shell and Greenpeace. We are saddened when the Lego brand is used as a tool in any dispute between organisations.

"We expect that Shell lives up to their responsibilities wherever they operate and take appropriate action to any potential claims should this not be the case. I would like to clarify that we intend to live up to the long term contract with Shell, which we entered into in 2011."

It remains to be seen how much impact Greenpeace's campaign will have on the Lego brand and sales, but Greenpeace says that "the campaign targeting Lego's deal with Shell launched on Tuesday 1 July, and already has more than 240,000 petition signatures".

Lego's bosses may well feel aggrieved. Sustainability site Triple Pundit reports that it has a proud green track record, pointing out that it invested $532m in a 32% stake in DONG Energy's wind farm which will provide enough energy to power Lego until 2020.

As Greenpeace's campaign gathers momentum, though, Lego may well be forced to reconsider its choice of partner. Expecting a partner to live up to their responsibilities doesn't really wash when you're accepting hundreds of millions of dollars from them.