Plaid Cymru are on course for their “best ever” election results, winning 19 seats in the Welsh Assembly, according to a BBC/ICM poll to mark St. David’s Day.

The poll projects the party winning 27% of the constituency vote and 25% of the regional vote, which if the swing was uniform would mean winning Blaenau Gwent, Caerphilly, Cardiff West and Llanelli from Labour and Aberconwy from the Conservatives.

Plaid Cymru’s best result so far was at the 1999 Assembly Election when they picked up 17 seats. In 2016 they won 12.

The poll, however, had better news for Labour than a YouGov poll earlier this week, putting their vote on 34% rather than 32% in the constituency ballot.

Assuming a uniform national swing since 2016 Labour would win 25 seats, the Conservatives 14, the Liberal Democrats one and UKIP one.

Constituency Regional Labour 34% 32% Plaid Cymru 27% 25% Conservatives 23% 22% Liberal Democrats 7% 6% UKIP 5% 6% Others 4% 9%

“This would, of course, be Plaid Cymru’s best ever National Assembly election result, and Labour’s worst,” Professor Roger Aawn-Scully of Cardiff University said of the results.

“Two things are immediately noticeable about these figures. The first is that ICM put Plaid Cymru in a rather better position than YouGov; this repeats the findings from last year, when – as noted here – Plaid did a bit better in polls from ICM and Sky Data than in near-contemporaneous polls from YouGov.

“Quite why this should be I am unsure. What we can be rather more sure about is that the 27 percent in this poll is Plaid’s highest score for the constituency ballot on any poll since an NOP poll early in the campaign for the 2003 National Assembly election.

“The second important thing to note is that these figures for the National Assembly also show a significant Labour decline on the last such ICM poll a year ago.

“Then Labour was on 40 percent support on the constituency ballot. There seems little doubt that Labour support has fallen, even if we are unsure about the extent of the decline.”

ICM Unlimited interviewed a representative sample of 1,001 adults aged 18+ by telephone on 7-23 February 2019.

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