Zac Goldsmith is struggling to keep up in the race to be London mayor

A new poll published on Monday has given Sadiq Khan an eight point lead in first preference votes over rival Zac Goldsmith in the race to become the next mayor of London.

With just over four weeks until Londoners cast their votes, an exclusive Opinium survey for the Evening Standard says Labour candidate Khan still enjoys a sizable lead and is more trusted than Conservative Goldsmith on the majority of key issues.

According to the survey, London voters have greater confidence in Tooting MP Khan in all aspects of public transport, as well as housing, crime reduction and unemployment.

Opinium estimates the current figures point to a 54-46 per cent win for Khan on May 5. That's not good news for Goldsmith given the short space of time that remains until the election.

The race is far from over though. Goldsmith may not be exactly white-hot on Khan's tail, but in the outer reaches of the capital his popularity has steadily increased from 27 to 30%, and his total share of second preference votes is up on last month.

The MP for Richmond Park and North Kingston can also be encouraged by the steady ground he is making in policy areas. Goldsmith may still trail in all the areas surveyed bar the environment, business and airports, but across all the categories public trust in him is growing. For example, the percentage of voters who trust Goldsmith in the categories of "public transport fares" and "reliability" has risen five points since Opinium's last poll.

Importantly, nearly a quarter of respondents (24%) still don't know how they plan to vote on election day - meaning there is still a significant and potentially-decisive chunk of the vote to play for.

Speaking to the Evening Standard, Adam Drummond, head of political polling at Opinium, said:

While Sadiq Khan has a growing lead in the first round, Zac Goldsmith appears to be doing better among second preferences than before and to be gaining ground on who Londoners trust to handle key issues. The race is far from over.

Even with this in mind, Goldsmith still faces a massive fight if he is going to replace Boris Johnson as mayor.

It's made even more difficult by the fact that Khan's grip on central London is only getting stronger. Last month, 37% of central London residents backed Khan. This month, that has risen to 45%.

Khan has also seen his popularity in the two youngest demographics (18-24 and 25-34) increase in the last month.

Reacting to the poll, Khan told the Evening Standard:

The poll that matters is on 5 May. We will be fighting hard for every vote until 10pm on that day, putting forward my positive programme on the issues that matter to Londoners.

The campaign to become London's next mayor intensified last week when Khan and Goldsmith clashed over the issue of tube and bus fares.

The Labour candidate promised to freeze fares for four years and accused Goldsmith of planning to hike fares by 17%, while the latter responded by slamming Khan's fare-freeze plan as "reckless" and said it jeopardised the investment London needs to push forward with plans such as the expansion to the overground service.