The government has beefed up its plans for an environmental watchdog following widespread criticisms that they were too weak.

The House of Commons threw out an amendment to the EU Withdrawal Bill, passed by the House of Lords last month.

Conservative MPs backed an alternative amendment tabled by former Cabinet minister Sir Oliver Letwin, which proposed  more teeth for the agency – the key concern of critics following the publication last month of a consultation paper outlining how the new watchdog will work.

The amendment equips the new agency with powers to initiate enforcement of environmental breaches as well as stipulating that it will have to abide by environmental principles, such as the ‘polluter pays’ principle.

Robert Buckland, the government’s Solicitor General, told the Commons that the powers would be ‘proportionate and appropriate’, and used only as a ‘provision of last resort’.

He said further details of how the principles outlined in the amendment would be interpreted and applied will be fleshed out in a draft Bill and forthcoming policy statement on post-Brexit environmental safeguards that ministers have promised.

But Mary Creagh, chair of the Commons environmental audit committee, criticised the government’s latest proposals for not guaranteeing the new watchdog’s independence while welcoming ministers’ concession on enforcement powers.

She said: “Previous governments have form on abolishing environmental watchdogs whose criticisms of government are a little too uncomfortable and tart. We do not want to set something up only for a future government to shut it down.”

The new body should have “the form and funding necessary to do the job, protected from ministers who might want to muzzle it in the future”.

Green MP Caroline Lucas agreed with Creagh that the new agency should be fully independent.

She said: “We may well end up with a green poodle, not a green watchdog.”