Here’s how the parties’ poll averages for May compare with those for the previous four months:

Labour finished May on 34.9%, down 1.7 percentage points on its average for April. The Tories were unchanged on 32.4%. Ukip were up 0.8 points to 14.8%, while the Liberal Democrats slipped to 8.8%: a fall of 0.7 points, and the party’s lowest monthly score of this parliament.

There hasn’t been an awful lot of movement in these averages since my previous poll round-up in mid-May. Both Labour and Ukip are exactly where they were just over two weeks ago. The Tories’ figure of 32.4% represents a tiny fall on their mid-month average of 32.7%, but to exactly the same point they were at the end of April.

It’s only the Lib Dems who have seen their average undergo a change in a consistent direction. They ended April on 9.5%, had reached 9.1% by mid-May, and concluded the month on 8.8%.

The party hasn’t had a monthly average as low as that since the 2010 general election (and possibly quite a few years before that).

By extending the graph backwards to cover a span of 12 months, you can see Labour’s recent decline and Ukip’s spurt in a wider context:

Ukip has yet to reach its all-time high average, which it scored in May 2013 (15.9%).

And while Labour’s average for the end of May is unchanged from that of the middle of the month, the party is still down three points in as many months.

But it’s worth remembering that Labour would still form the next government on these current averages. Indeed, the party could slip to as low as 31% in the polls and still end up in power, so long as the Tories remain two or three points behind them.

And for the Tories to be even the largest party in hung parliament, they need to be at least three points ahead of Labour.

June’s opinion poll trends should show things settling down again after the turbulence of the local and European elections. I suspect Labour would be relieved if its trend settled down where it is now, around 34-35%, curtailing the decline of the past few months.

But I imagine the Lib Dems would be anything but relieved were its trend to settle down below 9%.

Any lower and the party will be officially in fifth place in the opinion polls, behind “others”.