The fierce byelection battle in Mid Bedfordshire is poisoning relations between Labour and the Lib Dems and risks denting informal cooperation to remove the Tory government, senior Labour figures have warned. An incredibly close three-way battle has emerged in the seat formerly held by Boris Johnson ally Nadine Dorries, who quit after being refused a place on the former prime minister’s peerages list. Labour and the Lib Dems are both convinced they have the better chance of overturning the colossal 24,664-vote majority and pulling off one of the biggest byelection wins in history this week. While no formal agreement has been made between the parties, officials in Labour and the Lib Dems have effectively diverted resources away from byelections in which the rival party is better placed to unseat the Tories. It has contributed to Labour wins in Selby and Ainsty, as well as Lib Dem victories in Amersham and Chesham, and Somerton and Frome. Party figures supportive of tactical voting hoped it could be a sign that both leaderships could adopt similar tactics at the general election next year. However, there are concerns that the clashes in Mid Beds could disrupt the unspoken arrangement. “You can see what has been going on,” said one influential Labour figure. “Just look at what has been happening in the past. Mid Beds aside, we’ve had small campaigns where they are stronger. Some of those tactical and funding decisions will be very important at the election. Mid Beds is creating a problem.” With just days left until votes are cast, the two parties are engaged in an increasingly bad-tempered battle. Labour reacted with fury to a Lib Dem suggestion that their candidate lacked the “traditional values” of the area – while the Lib Dems have deplored a Labour attack ad suggesting Ed Davey’s party backed putting “fake missiles in nuclear submarines”. Canvassing has been so competitive that activists say they have had to queue up to speak to individual voters. There have been battles over the planting of party flags in the gardens of willing householders. Lib Dems had been trying to convince Labour that it had a better chance in Tamworth, where a byelection is taking place on the same day. Labour held Tamworth in 2005. Labour says polling shows it is neck and neck with the Tories in Mid Beds. One Labour MP who went to canvass in Mid Beds came away less inclined to back plans that would clear the way for the Lib Dems in certain seats. “In the past, I’ve gone to a nearby seat where the Lib Dems have a better chance than us and I’ve made the difficult argument to the local party that they are needed elsewhere during the election,” they said. “Understandably, they don’t like it. But why would we do that now, with the tactics the Lib Dems are using?” Lib Dem strategists said that whatever the tensions in Mid Beds, the seat was “an anomaly” that would have little relevance by the time of the election. “There’s a skirmish at a low-ranking level in one particular seat,” said a party source. “It is exceptional, but some members of the Labour party don’t see us as a separate party. I can count on one hand – three fingers, in fact – the seats where we’re in second place and running a campaign facing Labour for the general election. People need to hold their nerve, as there’s a much bigger prize here. And the biggest prize of all is, of course, getting the Conservatives out of power. “By the time we get to the general election, folk will know what’s needed and necessary to get rid of this government. I don’t think this is complicated or difficult.” A three-way split could see the Tories win in Mid Beds, even if they have half the vote share that they enjoyed at the last election. One Conservative MP said that a Tory victory could actually galvanise Labour and Lib Dem cooperation. “The best thing that could happen to them is if the Tories win it,” they said. “At that point, there will be a big focus on tactical voting at the election.”