With a week to go before the general election, the poll showed the Conservatives' share of the vote had fallen by four percentage points since last week to 44 percent, while Labour's had increased by three points to 36 percent.
Support for the Liberal Democrats was unchanged at 7 percent, according to the poll taken between 26 May and 1 June.
That left May with an eight-point advantage compared with 15 points in a poll that closed on 23 May.
The poll is the latest to show a narrowing lead for May. A YouGov opinion poll on Wednesday showed the Conservatives' lead for the 8 June election had fallen to a fresh low of 3 percent.
A failure to win the 8 June election with a large majority would weaken May just as formal Brexit talks are due to begin, while the loss of her majority in parliament would pitch British politics into turmoil.
Panelbase changed its methodology in its previous survey - it now includes only those certain to vote. It also tweaked its weightings for age and regional turnout.
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