A landmark Climate Change Bill, which for the first time sets a legal requirement on a government to cut carbon emissions, is expected to pass its first parliamentary hurdle on Monday but has a rocky ride ahead.

The House of Lords is expected to vote in its third reading on Monday on the bill which, in a departure from normal practice, was introduced in the upper house of parliament before the House of Commons to try to speed up the legislative process.

The government had hoped to get the bill into law by May, but amendments forced through in its passage through the House of Lords against strong government opposition could now delay that until October or November, climate campaigners said.

"It will be very tight to get it through this parliamentary session," said Friends of the Earth campaigner Martin Williams. "It is possible. But it is more likely it will be the Autumn."

The bill forces the government to cut climate changing carbon dioxide emissions by 28-32 percent by 2020 and 60 percent by 2050 with five year rolling "carbon budgets".

Climate campaigners have pushed for the end target to be raised to 80 percent, a figure the government has said it will ask a special climate committee set up by the bill to look at by the end of the year.

In its passage through the House of Lords environmentalists managed to get several amendments put in to strengthen the legislation.

"This is a much better bill than it was when it was introduced," said Williams. "But that also means it may face a rough ride when it goes to the House of Commons."

But the almost certain rejection of many of the amendments by the Commons means it will have to come back again.

The amendments shift in part the burden of responsibility for compliance from the environment minister to the prime minister and set annual indicative targets within the five year carbon budgets.

Campaigners have failed to get the 2050 target raised to 80 percent but claimed partial victory with the inclusion of a clause committing the government to hold temperature rises to two degrees -- a goal they say is equivalent to 80 percent.

In what they hailed as a major victory the amended bill also sets a strict limit on how many carbon emission permits the government can buy in from abroad to cover any shortfall in the national reduction performance.

The government had fought against a limit on foreign carbon credits which would give it broad leeway to underperform its own legal targets.

"The Lords' amendments do improve the bill, which means many won't make it through the Commons," said Greenpeace climate campaigner Charlie Kronick.

The European Union's Emissions Trading Scheme has turned carbon into a commodity through issuing emission permits which can be bought and sold by companies.

Also under the Kyoto climate change accord's Clean Development Mechanism countries and companies can buy into low emission developments to offset their own emission overshoots.

Scientists say global average temperatures will rise by between 1.8 and 4.0 degrees Celsius this century due to carbon emissions from burning fossil fuels for power and transport, causing floods and famines and putting millions at risk.