All parties have suffered financially during the pandemic, which has made it harder to raise funds through events for donors and dinners with party members.
But Labour, which is more dependent on regular donations from the trade unions, is likely to have seen a smaller shortfall.
A senior Conservative said: "The last event we held was the winter party in February last year. It’s been really challenging for fundraising."
Full party accounts for 2020 are not yet available, but Electoral Commission records show the Tories took in £15.5 million in large donations over the course of the year, down around one third on 2018 – the last year without a general election, which tend to boost the coffers of all parties.
Parliament has agreed to go on recess in late September and early October, which is traditionally the time for party conferences to take place.
Labour is holding its conference in Brighton as normal, telling activists: "It is our continued hope and focus that we will be able to meet in person. However, we will work in line with the government restrictions at the time."
The Liberal Democrats have cancelled their in-person gathering rather than taking the risk that it would be called off at the last minute.
The Conservatives have deliberately withheld their plans ahead of the repeal of England's remaining Covid-19 rules on Monday, on the grounds that announcing a decision on their conference would be seen as pre-empting the Prime Minister's lockdown road map.
But in coming weeks, the party will announce to supporters that its conference will go ahead in Manchester, with some additional virtual participation to enable as many people as possible to get involved.
A source said: "We want to use the hybrid format to make things more exciting and more interactive."
Last year, all party conferences were cancelled as cases started to rise again over the summer and autumn while uncertainty over the timing of vaccine availability remained.