Ministers are facing a fortnight of showdowns with peers over weak post-Brexit green protections just weeks before the Cop26 summit on the environment.

An alliance of crossbench and opposition peers has tabled more than 100 amendments to the environment bill in an attempt to beef up protections for nature, air quality and water standards and give the new green watchdog more powers.

With four sessions of parliamentary debate over the next two weeks, ministers may be in the position of arguing in favour of reduced domestic environmental standards while trying to claim a global leadership role before the Glasgow climate conference.

If the government is defeated, the legislation will ping-pong between the Commons and the Lords, a process that threatens to become more embarrassing for ministers the closer it gets to the November summit.

The bill, which is in its final parliamentary stages in the House of Lords, is intended to replace EU environmental regulations and will create a new Office for Environmental Protection. Several amendments will focus on trying to give the green watchdog far stronger powers than the government has planned.

It comes as Nicola Sturgeon addressed reports that No 10 was seeking to sideline her during the upcoming summit.

Responding to the leaked messages reported by the Independent, Scotland’s first minister said on Twitter: “All that matters is that Cop26 delivers an outcome to meet the Paris Agreement of limiting global warming to 1.5 degrees.

“We must work together and maximise contributions towards that. Anyone – me or PM – who allowed politics to get in way would be abdicating that responsibility.”

A year after the EU referendum, Michael Gove, then the environment secretary, pledged to deliver a “new, world-leading body to give the environment a voice and hold the powerful to account”.

Green campaigners say the OEP will be toothless compared with the European court of justice, which was instrumental in enforcing environmental protections such as cleaner bathing waters and better protection for marine life while the UK was a member.

In one of its final judgments before the UK left the EU, the court ruled that the government had “systematically and persistently” broken legal limits on air pollution, which hastens the deaths of 40,000 people a year.

But the OEP will not have any independent power to issue binding judgments or force the government to take action. It would be able to refer cases to the high court, but ministers could intervene via a process called “environmental review”, allowing them to give “guidance” to the OEP, according to Greener UK.

Ruth Chambers of the Greener UK coalition said: “The stronger the OEP is, the more chance we have of reducing air pollution and enhancing water quality.

“So instead of battling to weaken our environmental protections, ministers can use the next two weeks to improve the bill and create some positive momentum ahead of Cop26.”

Lord Krebs, the crossbench peer who is leading an amendment that will remove the ministerial “guidance power” and enhance the body’s independence over appointments and funding, said: “Without a powerful and independent watchdog to hold the government to account, if necessary through the courts, there’s no guarantee that the warm words about improving our environment will be matched by real action.

“With support from across all parties in the Lords, I’m asking the government to give the new watchdog proper independence from ministerial interference.”

Lord Anderson, the crossbench peer leading efforts to increase the power of the OEP and court, said: “The OEP can take public bodies to court for breach of environmental law – but the process is byzantine and the court is barred from ordering remedies that could harm the interests of developers or landowners.

“So it’s unlikely to bother, save in exceptional cases. If the OEP is to be an effective enforcer, the courts need to be given back their teeth.”

The government has already made concessions by promising measures to set a legally binding target to halt species decline by 2030, as well as stronger planning guidance.

Beccy Speight, the chief executive of the RSPB, said she was “pleased to see that our politicians are recognising that this government must not only start the work in tackling the nature crisis but also commit future governments to action.

“We need a powerful and independent environment watchdog to see this legacy delivered.”

Boris Johnson is keen to create a “global Britain” brand after Brexit, and has claimed 2021 will see “a year of British leadership” with the 26th UN climate change conference in Glasgow in November.

The government has said that the bill will hold ministers to account, and it contains measures including legally binding targets on air pollution.

It added that the OEP would be “operationally independent” from Defra and ministers would not be able to “set its programme of activity, or improperly influence its decision-making”. It added that the OEP would have powers to take legal action in serious cases.