In opposition: 2010–2024
Gardiner was re-elected at the 2010 general election with a majority of 8,028 votes. Upon his re-election, he was described by Andrew Roth in The Guardian as "One of the best educated and most internationally experienced MPs". He nominated David Miliband in the 2010 Labour Party leadership election.
Between 2011 and 2013, Gardiner served as Ed Miliband's Special Envoy for Environment and Climate Change. In June 2013, Gardiner and Conservative MP Tim Yeo jointly tabled an amendment to the Energy Bill which proposed establishing by 2014 a decarbonisation target for the UK's electricity generating sector, to be achieved by 2030. The amendment was narrowly defeated.
Gardiner was appointed as Shadow Minister for Natural Environment and Fisheries in July 2013. In this role, Gardiner criticised the lack of prosecutions of leading players ten months after David Cameron promised that everything possible would be done to deal with crime relating to the horse meat scandal. "The extraordinary thing is that because of its clout, industry has been able to commit what appears to be a criminal offence – selling the public horsemeat falsely labelled as beef – and just say they are sorry and didn't know". He gave his support in 2014 to the first annual Hen Harrier Day demonstrations in Derbyshire to highlight the illegal persecution of UK raptors. In January 2015, he admitted the push by the previous Labour government to encourage car-buyers to opt for diesel vehicles in a bid to protect the environment was "wrong", identifying that a "massive problem for public health" had been created.
When Jeremy Corbyn was elected Leader of the Labour Party in September 2015, Gardiner moved to becoming Shadow Minister for the Department of Energy and Climate Change. Following a string of Shadow Cabinet resignations in June 2016, Gardiner replaced Lisa Nandy as Shadow Secretary of State for Energy and Climate Change. In June, the department was abolished and he became Shadow Secretary of State for International Trade and Shadow Minister for International Climate Change.
Gardiner supported the development of Hinkley Point C nuclear power station, rejecting criticism of Chinese involvement in the project despite military and intelligence figures warning ministers that such involvement posed a threat to national security. He urged Theresa May to tell China that Britain wanted strong investment in infrastructure projects and described her decision to halt the deal as "politically stupid" and tantamount to "closing UK Plc down". He criticised May for negotiating a "rip-off deal" over its development.
In October 2017, Gardiner became involved in a complaint by Boeing to the US Department of Commerce. Boeing claimed Bombardier Aerospace was 'dumping' aircraft in the US by selling them below the cost of production, due to large Canadian government subsidies. The US Department had suggested the solution was a 300% tariff on Bombardier CSeries being sold to Delta Air Lines, which would adversely affect prospects for the sale. A major employer in Belfast, a punitive tariff again Bombardier would threaten more than 4,000 manufacturing jobs. Boeing's complaint against Bombardier had made no reference to its own multibillion-dollar tax breaks from Washington state. Gardiner seized on this and accused Boeing of hypocrisy, insisting all aircraft manufacturers require government subsidies; he labelled Boeing "the king of corporate welfare" and a "subsidy junkie", and suggested the company was trying to "crush a competitor". Boeing replied their illegal-subsidies complaint against Bombardier was about selling aircraft below the cost of production and not anti-competitive, saying it merely wanted "fairness" in "following trade rules".
In response to the murder in 2018 of Saudi journalist Jamal Khashoggi, Gardiner said "We must look very carefully again at the relationship we have with Saudi Arabia. What we would do certainly at the moment, and I think the government should do this, is to suspend all arms sales to the kingdom."
In January 2020, Gardiner received India's Padma Shri award for his work in the field of public affairs. He endorsed Rebecca Long-Bailey in the 2020 Labour Party leadership election. Following Keir Starmer's victory in the contest, he was stood down from the Shadow Cabinet in April of that year.
After following government social distancing guidelines from March to June 2020 during the COVID-19 pandemic, Gardiner joined Black Lives Matter protesters in London as they marched outside Parliament. Since the protest had ignored government guidelines, he took a test for COVID-19 but tested negative for the virus.
On the backbenches, Gardiner began organising a cross-party effort to bring in legislation banning fire-and-rehire practices by employers. In this role, he helped the Labour whips office to organise the defection of Conservative MP Christian Wakeford to the Labour Party in January 2022.
