Ukip is stretching its lead in the Rochester and Strood by-election and is picking up growing support from disgruntled Labour voters, according to a poll seen exclusively by the KM Group.

The poll will heap pressure on the Conservatives, who are desperate to avoid a second successive by-election defeat next month but increasingly look like it will lose.

The poll, being released tonight, was conducted by Survation on behalf of Unite and follows two earlier ones that also suggested that Ukip is odds on to win the crucial by-election.

The latest poll shows that Ukip is on 48%, eight points up on the previous poll and the Conservatives on 33%, up two points.

Labour is on 16% - a dramatic drop of nine points; the Liberal Democrats on just 1% - down two points and the Green party on 2% - up 2 points.

The poll of 1,012 people in the constituency, was conducted between October 27 and 28 and follows a series of high-profile visits by the Conservatives to rally support for their candidate Kelly Tolhurst.

It also has worrying findings for Labour, which has been criticised for a lack-lustre campaign.

Of those who supported Labour in 2010, 33% say they are intending to vote for Ukip, indicating that it is not just disaffected Conservatives who are being won over to Nigel Farage's party.

For the Conservatives, 41% of those who supported the party in 2010 say they will vote for Mark Reckless.

The same poll revealed that the NHS was the most important issue for voters in the constituency, with 37 per cent of voters saying the quality of local NHS hospitals and GP services in Medway was the most important issue for them and their family.

Medway Hospital was placed in special measures for poor standards.

Unite general secretary, Len McCluskey, said: “The future of our NHS is what really matters to voters and their families in Rochester and Strood and it’s going to be what really matters to voters at the next general election.

"It’s the party that fights most for the NHS, not ‘the party that fights most against immigration’ that will win the next general election.”

Of those questioned, 76% opposed plans that could see American companies having a stake in providing NHS services under what is known as Transatlantic Trade and Investment Partnership (TTIP).

There is ominous news for the Liberal Democrats, whose poll rating of just 1% suggests they are likely to be pushed back by the Green party.

Read the latest by-election blog by KM Group political editor Paul Francis.