Theresa May has said her government is serious about improving the environment after pressure groups gave a lukewarm response to a 25-year green plan, praising its ambition but warning that it lacked sufficient proposals for immediate action.

May’s proposals were also criticised by Jeremy Corbyn, who said her pledge to stop all avoidable plastic waste by 2042 was “far too long” to take action.

Unveiling the plan at a nature centre in south-west London, May argued that preserving the environment was natural for Conservatives, and she reiterated a promise that the current generation in the UK would be the first to leave the natural world in a better state.

The PM spent a significant part of the speech, at the London Wetlands Centre in Barnes, highlighting the issue of plastics pollution, saying people in the future would be “shocked at how today we allow so much plastic to be produced needlessly”.

However, the only immediate new policy was to extend the successful 5p charge for plastic bags to smaller shops in England, as has already happened in Scotland and Wales.

Other measures will be consulted on, for example a possible charge on single-use plastic containers such as takeaway boxes, an idea to urge supermarkets to introduce aisles without any plastic packaging, and research funding for “plastics innovation”.

Answering questions from the media after the speech, May denied that the blueprint was bigger on vague promises than on immediate action, calling it “an inspiring plan”.

She said: “It is a long-term plan, it’s about the next 25 years, but it’s a plan which I think speaks to everybody who has an interest in our environment and everybody who wants to ensure that future generations are able to enjoy a beautiful environment.”

The government was already taking action “in the here and now”, she argued, highlighting the plastic bag ban and legislation to outlaw microbeads.

She rejected the charge that the plans put more emphasis on consumers than producers, and defended the lack of any immediate proposal for a plastic bottle deposits schemes.

Recounting how as a child she would receive sixpence for returning glass soft drink bottles, May said the issue was determining whether for plastics it was more effective to recycle or reuse. “But I think the important thing is to see what is going to have the greatest impact,” she said.

The wider plan, produced by Michael Gove’s environment department, spans 151 pages and covers areas including land use and soil health, tackling climate change and poor air quality, and encouraging better access for all to green spaces and nature.

It also outlines a plan announced at the weekend to create a 120-mile northern forest between Liverpool to Hull, and the creation of a “national tree champion”.

The plan is vague on how the goals will be measured and enforced, saying ministers will consult on a new independent body to hold the government to account.

The plan was met with some caution by environmental groups, and criticised by Labour. Corbyn said plastic waste and “the throwaway society” needed to be tackled more quickly than May’s timetable. “We have to be much, much tougher. Yes, take it on. But don’t do it in 25 years – do it now.”

Tanya Steele, chief executive of the charity WWF, said it was positive to hear May’s pledges. “But these commitments will only become a reality if they are backed by the force of law, money and a new environmental watchdog.”

John Sauven, Greenpeace UK’s executive director, said the British environment “needs a 25-month emergency plan more than it needs a 25-year vision”.

He said: “If Theresa May wants to persuade people this is more than just husky hugging, she needs to put some joined-up thinking at the heart of her strategy. You can’t claim to care about climate change and our countryside and then back fracking, or care about the next generation and then let air pollution harm our kids’ health.”

Michael Gove, environment secretary, speaks at the unveiling of the green plan on Thursday. Photograph: Daniel Leal-Olivas/AFP/Getty

Craig Bennett, chief executive of Friends of the Earth, said that while a long-term vision was vital, so was immediate action on areas such as air pollution. “It’s time to stop tinkering at the margins and get to the heart of the problems – especially the nation’s fossil fuels addiction,” he said.

Mary Creagh, the Labour MP who chairs the environmental audit committee, which called last week for a 25p “latte levy” on all disposable hot drink cups, said the plan “delays answering the hard questions over how to tackle plastic pollution and fails to provide any legal basis for its ambitions for the environment, which will be needed after we lose EU legal environmental protections after Brexit.”

The scale of plastic waste pollution around the globe has become an increasingly important issue. Last year, the Guardian revealed that a million plastic bottles were purchased every minute around the world, driven mainly by the consumption of drinking water. It also reported that tiny particles of plastic were now present in drinking water worldwide.

In her speech, May – who praised David Cameron for having “restored environmentalism to a central place in the Conservative agenda” – spoke at length about the problem of plastics pollution.

“In the UK alone, the amount of single-use plastic wasted every year would fill 1,000 Royal Albert Halls,” she said. “This plastic is ingested by dozens of species of marine animals and over 100 species of sea birds, causing immense suffering to individual creatures and degrading vital habitats.

“One million birds and over 100,000 other sea mammals and turtles die every year from eating and getting tangled in plastic waste. This truly is one of the great environmental scourges of our time.”

Asked about the impact of Brexit, May said it would not lead to lower environmental standards. “We will set out our plans for a new, world-leading independent statutory body to hold government to account and give the environment a voice. And our work will be underpinned by a strong set of environmental principles.”