By Jonathan Amos
BBC News science reporter
Thousands of jobs and the continued strength of the UK space sector hang on key financial decisions taken this week in Whitehall.
GMES would pool data from Earth observers such as Envisat
Ministers will finalise the levels of funding Britain will commit to major European space projects.
Weak promises of cash could see one of the UK's most highly skilled industries missing out on substantial contracts.
"We're at a critical stage for investment and return in UK space," said Conservative MP Ian Taylor.
"If we don't play our part, we might find our skills and industrial base start to evaporate," the former science minister and co-chair of the Parliamentary Space Committee told BBC News.
Ian Waddell, the national aerospace officer for the Amicus union, added: "We have a week to persuade the government to come up with its appropriate share, a week in which to protect the 15,000 highly skilled jobs in the UK space industry."
Member states of the European Space Agency (Esa) meet in Germany next Monday for two days of hard-bargaining and decision-making.
The pivotal gathering will sign off a number of prestige projects, such as a mission to put a lander on Mars.
WHAT IS THE GMES PROJECT?
A joint initiative of the European Union and Esa
Pulls together all Earth-monitoring data whether collected in space or not
Will use existing and newly commissioned spacecraft
Crucial to the understanding of how our climate is changing
Important for disaster monitoring - earthquakes, floods, eruptions, etc
An enforcement tool for EU policies: fishing quotas, etc
The European component of a global project known as Geoss
But national governments will also come to the meeting with pledges of cash for new ventures - one of which is central to British interests.
Called GMES (Global Monitoring for Environment and Security), this flagship programme will build a coordinated system for Earth observation and monitoring.
It will cost some 2.3bn euros (£1.6bn) in total and pull together all the data obtained by environmental satellites, air and ground stations; to provide a comprehensive picture of the planet.
GMES will play a primary role in our understanding of climate change, and, as such, was championed by Prime Minister Tony Blair at the Gleneagles G8 summit back in July.
For the UK to take a leading role, it will have to "subscribe" about 35m euros (£24m) over two years to the first phase of the project, with the Department for Environment (Defra) providing most of the money.
This will not only allow Britain to shape GMES but also put its space companies in pole position to win the contracts that are handed out at a later date to develop the project.
"The UK space industry has built up a very strong capability in the geo-information sector," explained Dr Barbara Ghinelli from EADS-Astrium, a satellite manufacturer based in Stevenage and Portsmouth.
"GMES will help in expanding this capability and make sure we have an increase in space industry jobs and skills."
But MPs speaking in the House of Commons last week said they were concerned to hear that ministers might decide to subscribe a much smaller sum, perhaps only £6m.
And under the system of juste retour, or fair return, which is operated on European budgets, this could mean far fewer contracts coming back to the UK.
"If we marginalise ourselves in some of these programmes then it will probably lead to a reduction in the critical mass of UK space activity," warned Ian Taylor.
The Labour MP for Portsmouth North, Sarah McCarthy-Fry, is of the same opinion.
"As far is GMES is concerned, it is not as if we're not talking huge megabucks [for the initial contribution]," she said.
"If we're going to keep any manufacturing in this country, it's going to be in the high-skill, hi-tech sector - exactly what space jobs are - and it would be very short-sighted if we cut our contributions."
One fear being expressed is that any shortfall in GMES funding could yet be made up by taking money away from other projects.
A noted concern is Artes (Advanced Research in Telecommunications Systems), another Esa programme that puts money into basic research and product development.
The Inmarsat-4 series benefited from the Artes programme
It gets its UK contribution - about £20m - through the Department of Trade and Industry (DTI). It helped fund, for example, the early work that led to the electronics "brain" put in the Inmarsat-4 series of telecommunications satellites.
"The underlying problem to all this is that there isn't a coherent space policy in the UK," said Ian Waddell.
"The DTI's contribution is only one part of the total UK space budget; and the other departments don't see space as being that important to them in the grand scheme of things."
Sarah McCarthy-Fry has put down questions this week for ministers in both the DTI and Defra, asking them to clarify their positions on Esa contributions. "We want to make sure they are aware of the implications," she said.
Spokespeople for Defra and DTI told BBC News no final decisions had yet been made on the GMES and Artes contributions.
And on Monday, UK science minister Lord Sainsbury spoke at an EU-Esa Space Council meeting at which he emphasised the importance of GMES.
"Environmental monitoring is more important than ever before," he said.
"GMES has the potential to bring together existing and new technology - helping us to better understand and protect our planet."