Prime Minister David Cameron has once again been accused of failing to deliver on his pledge to lead the "greenest government ever" as green NGOs and businesses stepped up calls for the Conservative party to reinvigorate its environmental agenda at its annual conference in Manchester.

The conference kicked off yesterday with the launch of a new pamphlet on "modern, compassionate conservatism" and a session on the environment and climate change.

Writing in the foreword to the pamphlet, Cameron reasserted his commitment to modernising the Conservative party, insisting that "as a party, we have changed the way we look, what we do and the things we think about. We have brought a new focus to social action, development, the NHS and the environment."

Meanwhile, environment secretary Caroline Spelman delivered her main address to the conference, announcing plans for a new drive to tackle tree diseases and arguing that the government is "prioritising the needs of the countryside, after over a decade of neglect".

"This is a government that is prepared to act – and act in the interests of the long term," she said. "Nowhere is it more vital than for the environment. Our generation has a duty to restore it for the next. Our children, and theirs, depend on us getting it right. And, what's more, this government and this country should be leading by example."

However, she faced criticism for glossing over the government's aborted forest sell-off, continuing to pursue a badger culling policy that campaigners have branded unworkable and failing to announce any significant new policies.

The environment session was also overshadowed by fresh criticism from Friends of the Earth of the government's green record and confirmation from the Department for Transport that initial assessments suggested its controversial plan to increase motorway speed limits would result in an increase in carbon emissions.

Friends of the Earth's policy and campaigns director, Craig Bennett, said the government was failing to adequately promote green policies, while introducing new policies that will damage the environment.

"Despite all the rhetoric about championing the environment, the reality is that Conservative policies fall far short of delivering David Cameron's pledge to lead the greenest government ever," he said.

"Funding cuts for community green energy projects, a planning free-for-all in our countryside and encouraging councils to abandon successful recycling initiatives are just three examples in a long list of policy failures."

Meanwhile, the Guardian reported that the Department for Transport has undertaken an "initial analysis" of transport secretary Philip Hammond's plans to increase the top speed limit to 80mph, showing that road deaths would likely rise by about one per cent a year, while automotive emissions would also increase by one per cent a year, leading to a rise in annual emissions of 670,000 tonnes.

However, a recent report from the Committee on Climate Change suggested the increase in emissions that would result from a change in the speed limit could in fact reach 2.2 million tonnes a year.

The Department for Transport said it would undertake a full impact assessment of any change to the speed limit when it launches the formal consultation on the proposed changes.