By Kylie MacLellan
LONDON (Reuters) - Britain's opposition Labour Party is too weak to win a national election and should look to form an alliance with other smaller parties in order to have a chance of regaining power, a Labour-supporting think tank said on Tuesday.
Beset by internal rifts, the party has struggled to articulate a clear position on Brexit which it had campaigned against, giving Prime Minister Theresa May's ruling Conservatives a freer ride as she plots Britain's divorce from the EU.
Opinion polls consistently put Labour about 10 percentage points behind the Conservatives and if replicated at the next election due in 2020, Labour would win less than 200 seats in the 650-seat House of Commons for the first time since 1935, the left-wing Fabian Society said.
With polling in the past overstating Labour's popularity, it could actually win as few as 140 seats, the think tank, one of Labour's original founders, said in an analysis paper entitled "Stuck".
"For the time being Labour has no realistic chance of winning an election outright," said Fabian Society General Secretary Andrew Harrop.
"It is much more plausible to imagine a group of anti-Conservative parties securing sufficient votes to form a governing alliance ... although even this would still require a very large reversal in Labour's present fortunes."
Labour leader Jeremy Corbyn, a veteran socialist, was re-elected in September after a challenge from one of his lawmakers that exposed sharp divisions between the party's elected representatives and grassroots supporters.
On Monday, the head of Britain's biggest trade union and Labour's largest financial backer was reported as saying he did not think Corbyn would seek to cling on to power if the party's opinion poll ratings were "still awful" in 2019.
Commentators say Labour's weakness is giving May breathing space on her plans for Brexit about which she has so far given few details.
It has also meant her attention has been on addressing divisions in her own party between those who want a "hard Brexit" with a focus on curbing immigration and those who want Britain to remain in the EU's single market.
However, despite its problems and the fact only half of those who voted for the party in 2015 say they support it today, the quirks of Britain's first-past-the-post electoral system mean Labour will not die out, Harrop said.
"Labour is too strong to be supplanted by another opposition party; and too weak to have any realistic chance of governing alone ... The question now is whether the party can move forward, not back?" he said.
Labour's poor ratings and a court battle over whether parliament's approval is needed to begin EU divorce talks have increased speculation that May, appointed prime minister after June's Brexit vote, could seek to boost her slim parliamentary majority by calling a snap vote.
Labour already face an upcoming electoral test after lawmaker and vocal Corbyn critic Jamie Reed, whose northern English constituency voted strongly in favor of Brexit, said he would step down this month.
In a December election for a vacant Conservative-held parliamentary seat, Labour slipped from second to fourth place.
The Fabian Society said Labour faced a "Brexit dilemma" with leave supporters flocking to May's Conservatives and remainers to the pro-EU Liberal Democrats.
"Labour needs to be the party for the millions of voters who were neither die-hard remainers nor leavers," said Harrop.
(Reporting by Kylie MacLellan; editing by Michael Holden)