Analysis suggests five seats – including three held by Tories – are likely to fall to Ukip, but another 25 are vulnerable

Ukip has a chance of winning in at least 30 constituencies at the next election, although it is likely to win only in five, according to data compiled after the party’s byelection win in Clacton.

An analysis based on polling and other recently published data from the Guardian’s data team found the three Conservative seats of Boston and Skegness, Thurrock, and Thanet South – where Nigel Farage is standing, among the five most likely to fall to Ukip in 2015. The others are the recently taken seat of Clacton and the Labour seat of Great Grimsby.

The figures also suggests there are a further 10 seats where Ukip is a good contender, including the Conservative seats of Great Yarmouth, Camborne and Redruth, Bognor Regis and Littlehampton, Folkestone and Hythe, Thanet North, and Waveney, plus the Labour seats of Dudley North, and Rotherham. The Lib Dem seat of Eastleigh, and Rochester and Strood, the vacant seat of Tory-to-Ukip defector Mark Reckless, are also in this category.

In a sign of how hard the Conservatives will fight Reckless for Rochester, David Cameron has this week written to all constituents in the seat offering them the chance to select one of two potential candidates. They are local councillors Anne Firth and Kelly Tolhurst. The byelection, it was announced on Tuesday, will be held on 20 November.

A further 15 seats have been classified as areas where Ukip is in with a chance of winning in 2015, including the Heywood and Middleton seat where the party narrowly came second to Labour in last week’s byelection, and several Lib Dem southern coastal seats including Eastbourne and St Austell and Newquay.

The analysis suggests Ukip still poses the biggest threat to Conservative-held seats, but several Labour and Lib Dem MPs cannot rest on their laurels.

The biggest opportunities for Ukip appear to be in Boston and Thanet South, as two moderate Conservatives are stepping down in these constituencies, leaving them without incumbents. However, many MPs contesting Ukip in the tightest seats are still dismissive of the party’s prospects. Austin Mitchell, the retiring Labour incumbent in Great Grimbsy, said he would be horrified and astonished if Ukip’s candidate, Victoria Ayling, won in his seat. The longstanding MP, who some have mooted as a possible defector to Ukip, said: “Victoria is a good candidate and she works very hard but she won’t win. Grimsby is a Labour seat, it will stay a Labour seat and its interests lie with the Labour party. I don’t think Ukip is going to make any big impression. Their vote may go up, but so will Labour’s because we lost votes last time to the Liberals and that is coming back to us.”

Jackie Doyle-Price, the Tory MP for Thurrock, argues that Ukip will help to drag her “over the line” in her re-election efforts, saying it will help to take working-class votes off Labour.

Others in seats with strong Ukip support argue the threat is diminishing, not growing. Mike Thornton, the Lib Dem MP who narrowly fought off Ukip in the Eastleigh byelection of 2013, said his team had got better at responding to Farage’s party.

“We didn’t expect it in the byelection and we didn’t again in the county council elections, but in the more recent borough elections they didn’t get a single seat. Now we know what their message is going to be better, we know where they are going to target and we know better how to counteract their message,” Thornton said.

Because of the unpredictability of the fight, all the parties are braced for unexpected upsets. David Winnick, who has held the seat of Walsall North for Labour since 1979, is still ahead in his seat by five points, according to a survey by Lord Ashcroft, but the second party challenging him is now Ukip and not the Conservatives.

“If it wasn’t for immigration, Ukip would be in a very different situation … but my view is that bread-and-butter issues, frozen wages, midwives on strike, the anger and resentment that undoubtedly exist, Ukip are exploiting it to say it is more or less all immigration. Labour’s job is to say: yes, immigration is a problem and we don’t underestimate it, but the anger and resentment, we’re trying to demonstrate that it is to a very large extent due to Tory policies.” There have been numerous attempts by academics, thinktanks and pollsters to classify the seats that Ukip is most likely to win, but many of its local surges so far have taken the other parties by surprise. Ukip itself has a target list of at least 12 seats and possibly up to 25, including more challenging options such as Aylesbury, the safe seat of Conservative Europe minister, David Lidington, where there is significant opposition to HS2.

Farage’s party also has its eye on Portsmouth South, the seat of former Lib Dem MP Mike Hancock, who resigned over inappropriate advances to a female constituent.