By Andy Bruce and Elizabeth Piper

LONDON, Sept 6 (Reuters) - British Prime Minister BorisJohnson's plan to hike taxes to fund social care has provokedfury among many of his own lawmakers, who fear that such a clearviolation of his election promises shows he is happy to overseea sweeping expansion of the state.

After the fiscal splurge on the COVID-19 pandemic, Johnsonis now addressing Britain's creaking social care system, whosecosts will soar as the population ages, while facing numerousother thorny policy matters.

Johnson wants to raise the National Insurance (NI) tax paidby working people to subsidise care for pensioners, includingwealthy retirees, according to British media.

But many lawmakers from Johnson's own Conservative Partyworry this will hurt younger, low-income workers and breach his2019 election guarantee not to raise the rate of NI - whichstill adorns the Conservatives' website.

"The proposal will be aimed at supporting the more affluent,but the tax will be paid by lower income earners. That'sunfair," one Conservative lawmaker, who declined to be named,told Reuters.

"We shouldn't break solemn election promises unless thereare hugely good reasons to do so."

Like many other Western leaders, Johnson is facing demandsto spend more on welfare after government borrowing ballooned to14.2% of economic output - a level last seen at the end of WorldWar Two.

The row over his planned tax hike comes after charges thatBritain was unprepared for the chaotic fall of Afghanistan,while the country is also facing labour shortages and supplychain problems exacerbated by Brexit, as well as a high COVID-19death toll.

Johnson is due to address parliament on the Afghanistansituation later on Monday, and the BBC said he would shortlyannounce a new 5.5 billion pound ($7.6 billion) package for theNational Health Service. Details of the NI hike are likely tocome later, possibly on Tuesday.

'TAX-AND-SPEND'

For years, British leaders have been trying to find a way topay for social care without endangering support by hiking taxes.Johnson said he had a plan for social care in 2019.

British ministers are still thrashing out the details butJohnson had been expected to announce a roughly 1 percentagepoint increase on the rate of NI paid by workers and theiremployers, which official estimates show would raise around 10billion pounds a year.

"With regards to our plans for social care, we are committedto setting out long-term sustainable reform of the sector andthat is what we will do," Johnson's spokesman told reporters onMonday. The finance ministry declined to comment.

While Johnson's huge majority of 80 in parliament means adefeat may be unlikely, members of his own cabinet have hintedat their opposition to tax hikes.

House of Commons leader Jacob Rees-Mogg on Sunday invokedformer U.S. President George H.W. Bush, who came to regretsaying "Read my lips: no new taxes" during the 1988 electioncampaign.

"(Voters) remembered those words after President Bush hadforgotten them," Rees-Mogg wrote in a Sunday Express opinionpiece.

Many of Johnson's lawmakers fear raising taxes will alienatemany of the voters in northern England who supported him in the2019 election, when he promised explicitly not to raise incometax, Value-Added Tax or National Insurance.

BIGGEST EVER BENEFIT CUT

Johnson also promised in 2019 to maintain the "triple lock",which annually links the state pension to whichever is highestout of inflation, earnings or a 2.5% increase.

Owing to a statistical quirk during the pandemic, theofficial measure of earnings is running at almost 9% - whichwould result in a bumper payout to pensioners just as thegovernment has stressed the need to tighten belts. "Given that average wage levels have been skewed by theunprecedented events of the past 18 months, the (financeminister) should temporarily suspend the wages element of thelock," said Mel Stride, who heads parliament's TreasuryCommittee of lawmakers.

Johnson faces more widespread resistance to plans to removea 20 pounds-per-week increase to state benefits, known asUniversal Credit, that was introduced at the height of thepandemic. The move would affect 4.4 million households frommid-October.

"If the (government) goes ahead with this cut, it would bethe largest overnight benefit reduction that has ever happened,"researchers Torsten Bell, Adam Corlett and Daniel Tomlinson saidin a report published by the Resolution Foundation think tank onMonday.

"That should give policy makers reason to pause as towhether this is a good idea politically, economically ormorally." ($1 = 0.7225 pounds)

(Reporting by Andy Bruce; Editing by Hugh Lawson) ((andy.bruce@thomsonreuters.com; +442075423484; ReutersMessaging: andy.bruce.thomsonreuters.com@reuters.net))