LONDON (Reuters) - Britons still undecided on how they will vote in a referendum on EU membership think they would be 71 pounds ($103) a year worse off if Britain quits the bloc, a survey said, suggesting the "Leave" camp faces a bigger challenge than the "Remain" side.

Undecided voters, on average, estimated that households would be 181 pounds worse off per annum five years after a vote to leave the EU, compared with a hit of 110 pounds if Britain decided to stay, the survey showed.

Eighteen percent of the 3,000 respondents polled by market research firm Opinium and the European Centre for Research in Electoral Psychology at the London School of Economics said they did not know how they would vote in the June 23 referendum.

A quarter of those who have not made up their minds think that British households will be worse off after a vote to leave, compared with 17 percent who think they would be worse off if Britain stays in the bloc, the poll showed.

Polls on how Britons will vote in two weeks' time are too close to call. The "Leave" camp edged ahead in two polls on Monday, only for two separate polls to put "Remain" narrowly ahead hours later.

Thursday's survey also found voters were split evenly, although no detailed breakdown was provided.

It suggested voters across the board were more skeptical about the "Leave" camp's arguments. Respondents thought immigration would remain a challenge even after Brexit and the EU would be unlikely to offer Britain a better deal if it left. Opinium and the ECREP gave no definition of such a deal.

"Unless 'Vote Leave' can find more believable messages for those sitting on the fence, the large number making their final decision at the last moment could tip the scales in favor of Remain," Opinium managing director James Endersby said.

Voters were also skeptical about some arguments put forward by the "Remain" campaign, including suggestions that major banks would leave London if Britain quits the EU.

The poll found up to 30 percent of voters could make up or change their mind on how to vote in the last week of the campaign, and half of them could do so on polling day itself.

(Reporting by Freya Berry; Editing by William Schomberg and Louise Ireland)