By Andrew MacAskill and Elizabeth Piper

LONDON (Reuters) -British Prime Minister Keir Starmer was asked in parliament on Thursday why he had not appeared in front of lawmakers for the past two weeks. His answer was simple - he had been busy with international affairs.

The question underlines Starmer's awkward position - just a year after winning one of the biggest election victories in British history, his audience abroad is much less hostile than the one at home, where his party is divided over welfare reforms.

With Starmer's Labour Party lagging behind Nigel Farage's right-wing Reform UK in opinion polls and the prime minister's own popularity tumbling, more than 100 of his lawmakers are threatening the biggest parliamentary rebellion of his premiership to kill off the government's plans to reform the welfare system at a vote due next week.

On a trip to the NATO summit in The Hague this week, Starmer, 62, sat alongside U.S. President Donald Trump for talks and shared hugs and handshakes with other world leaders but was noticeably tetchy when asked if the rebellion brewing at home showed a lack of political judgment on his part.

"I am comfortable with reading the room and delivering the change the country needs," he said as he vowed to lead his party into the next national election, which is not due until 2029.

Downing Street officials say the vote is shaping up to be the biggest test of Starmer's authority to date. Were he to suffer his first parliamentary defeat, or were the government to pull the vote or make big compromises, his leadership would be undermined.

More fundamentally, the officials say the reforms to save money and encourage more people back into work are an essential part of what Starmer sees as a decade-long project to fix Britain's problems.

'VERY, VERY UPSET'

Annual spending on incapacity and disability benefits already exceeds the country's defence budget and is set to top 100 billion pounds ($137 billion) by 2030, according to official forecasts, up from 65 billion pounds now.

Ministers are in talks with lawmakers to offer compromises to ensure the legislation is passed next Tuesday. One key area under discussion concerns the number of people who would lose access to benefits designed to cover disability-related costs.

One of Starmer's ministers, speaking on condition of anonymity, told Reuters he thought a deal to see off the rebellion would eventually be reached.

But a Labour lawmaker, who also asked not to be named, said the government's best option would be to pause the legislation and rework it, adding that he would not vote for changes that could push tens of thousands of people into poverty.

"People are very, very upset at the way the government has handled this," the lawmaker said.

The decision to cut payments to some of the most vulnerable in society is particularly painful for politicians in the centre-left Labour Party, which founded the state-run National Health Service and traditionally sees itself as the protector of the country's welfare state built after World War Two.

Rebels may have been encouraged after the government reversed another unpopular policy - cutting payments to millions of pensioners to help them pay energy bills each winter.

Starmer told parliament on Thursday he wanted the "values of fairness" to be at the heart of his reforms and attempts to reach a consensus would continue over the coming days.

($1 = 0.7277 pounds)

(Editing by Gareth Jones)