Barack Obama was today expected to begin the process of reversing George Bush's policies on climate change by allowing individual US states to set their own, stricter standards for vehicle emissions.

The new president will instruct the federal Environmental Protection Agency (EPA) to look again at a request from California to be allowed to impose tougher limits on the emissions.

He could even instruct the agency to allow state opt-outs, according to reports in the US press. Obama's predecessor, George Bush, denied states this authority, prompting California and several others to sue.

Although any EPA decision would probably take several months, Obama's action would signal his intention to take tougher action on combating climate change, something he repeatedly promised during his election campaign.

If the EPA gives the go-ahead, up to 12 states could impose stricter emissions levels, with California hoping to reduce them by 30% by 2016.

The country's embattled carmakers have lobbied hard to prevent states being given autonomy over emissions, arguing that having different standards across the country would create confusion.

In a separate development, Obama is expected to direct the federal transport department to press ahead with revised US-wide efficiency rules for new vehicles made after 2011. These are likely to be weaker than many of the state standards.

Environmentalists welcomed the moves. "These actions are a clear demonstration that President Obama recognises the urgency of moving America to clean energy and tackling the climate crisis in 2009," Frances Beinecke, president of the Natural Resources Defence Council, told Reuters.

"President Obama, with these actions, will have done more for oil independence in one week than George Bush did in eight years," Daniel J Weiss, from the liberal thinktank the Centre for American Progress, told the Wall Street Journal, which also reported the new president's probable move.

The steps are the latest in a series of reversals of Bush policies Obama has made since taking office last Tuesday, beginning with a plan to close the Guantánamo Bay prison camp within a year and the lifting of restrictions on US government funding for groups involved with abortion overseas.

Separately, Obama faces a battle in piloting his planned multibillion-dollar economic stimulus plan through Congress after a weekend during which several senior Republicans expressed reservations about the proposals, which the president had hoped would attract bipartisan support.

John McCain, whom Obama defeated in November's election, said the plan did not go far enough. "There have to be major rewrites if we want to stimulate the economy," he said. "As it stands now, I can't vote for it," he told Fox News.

The Republican leader in the House of Representatives, John Boehner, added his concerns about the plan, which would involve spending more than $800bn (£585bn).

"I just think there's a lot of slow-moving government spending in this programme that won't work," he said. "We can't borrow and spend our way back to prosperity."

Obama's vice-president, Joe Biden said yesterday Congress needed to approve the measure quickly because "there's no good [economic] news on the immediate horizon".

Tomorrow, Obama will meet senior Republican Congressional leaders in private in an attempt to rally support behind the economic plan.